Eli Lilly and Software Engineer Interview Guide

The 60-second brief

As a Software Engineer at Eli Lilly and, you play a vital role at the intersection of life sciences, technology, and global healthcare innovation. Technology at Eli Lilly and is not merely an internal support function; it is a core driver of pharmaceutical discovery, automated lab operations, clinical trial execution, and advanced manufacturing platforms. Whether building high-throughput data processing engines within the Data Foundry, engineering control software for the Lilly Medicine Foundry, or creating scalable full-stack enterprise applications, your engineering contributions directly accelerate the delivery of life-saving medicines to patients worldwide. In this position, you will design, build, and maintain robust, enterprise-grade software systems that handle high complexity and stringent regulatory environments.

This guide is scoped to a Software Engineer candidate at Eli Lilly and.

Focus areas

• Behavioral Interviewing
• STAR Method (Situation-Task-Action-Result)
• Data Structures & Algorithms (DSA)

The Eli Lilly and interview process

Eli Lilly and candidates report 6 rounds over 4-6 weeks. The stages below are what candidates describe, not a published process.

Application Review

Initial review of applications or connections through university recruiting.

What to demonstrate

• Initial review of applications or connections through university recruiting
• Depth in Behavioral Interviewing

How to prepare

• Be able to walk your CV end to end in two minutes, and say why this company specifically.
• Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Recruiter Screen

Screening call with a recruiter to discuss qualifications and role fit.

What to demonstrate

• Screening call with a recruiter to discuss qualifications and role fit
• Depth in Behavioral Interviewing

How to prepare

• Be able to walk your CV end to end in two minutes, and say why this company specifically.
• Have your salary expectations, notice period and location constraints ready, and ask for the rest of the loop in writing.

Online Technical Assessment

Candidates may complete an online assessment to evaluate technical skills.

What to demonstrate

• Candidates may complete an online assessment to evaluate technical skills
• Depth in Behavioral Interviewing

How to prepare

• Answer aloud and timed: Give an example of a technical challenge you faced in a past role, how you approached it, and what you learned from the outcome.
• Answer aloud and timed: Given an array of integers, write an efficient algorithm to find all unique pairs that sum up to a target value.

Multi-Round Interviews

Candidates participate in several technical and behavioral interviews.

What to demonstrate

• Candidates participate in several technical and behavioral interviews
• Depth in Behavioral Interviewing

How to prepare

• Answer aloud and timed: Explain how you would optimize a solution from $O(N^2)$ to $O(N \log N)$ or $O(N)$ time complexity.
• Answer aloud and timed: How would you traverse and search a tree or graph structure, and what criteria determine whether you choose Breadth-First Search (BFS) or Depth-First Search (DFS)?

Technical Evaluations

Live coding, system design discussions, and resume probes are conducted.

What to demonstrate

• Live coding, system design discussions, and resume probes are conducted
• Depth in Behavioral Interviewing

How to prepare

• Answer aloud and timed: Discuss the trade-offs between using hash maps, balanced binary trees, and arrays for data retrieval in high-throughput systems.
• Answer aloud and timed: Explain memory management, stack vs. heap allocation, and how garbage collection impact software performance.

Behavioral Interviews

Interviews focusing on team collaboration, problem-solving, and value alignment.

What to demonstrate

• Interviews focusing on team collaboration, problem-solving, and value alignment
• Depth in Behavioral Interviewing

How to prepare

• Prepare three examples from your own work, each with a decision you made and an outcome you can quantify.
• Re-read the description of the behavioral interviews above and write down what you would ask to confirm before it.

Where candidates lose points

Standardize all your behavioral stories around the STAR format before your interviews

Clearly separate what the team did from what you individually delivered.

Thoroughly Know Your Resume

Interviewers will ask you to explain individual architecture choices, methodologies, and technical tools mentioned on your resume. Be ready to explain your direct technical contributions clearly.

Articulate Your Thought Process

During live coding or design sessions, think out loud. Explain the trade-offs of your approach before writing code, and proactively discuss runtime and memory complexity.

Research Eli Lilly and Values

Demonstrate genuine interest in the company’s corporate vision and commitment to healthcare innovation. Be prepared to answer "Why Eli Lilly and?" with concrete examples aligned with your career goals.

Questions & practice

Given an array of integers, write an efficient algorithm to find all unique pairs that sum up to a target valu

Medium · Data Structures & Algorithms

Given an array of integers, write an efficient algorithm to find all unique pairs that sum up to a target value.

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. State the target complexity and say which constraint rules the naive version out.

Follow-up

• How does this change if the input no longer fits in memory?
• What is the worst case, and how likely is it on real data?

Explain how you would optimize a solution from $O(N^2)$ to $O(N \log N)$ or $O(N)$ time complexity.

Medium · Data Structures & Algorithms

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. State the target complexity and say which constraint rules the naive version out.

Follow-up

• How does this change if the input no longer fits in memory?
• What is the worst case, and how likely is it on real data?

How would you traverse and search a tree or graph structure, and what criteria determine whether you choose Br

Medium · Data Structures & Algorithms

How would you traverse and search a tree or graph structure, and what criteria determine whether you choose Breadth-First Search (BFS) or Depth-First Search (DFS)?

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. State the target complexity and say which constraint rules the naive version out.

Follow-up

• How does this change if the input no longer fits in memory?
• What is the worst case, and how likely is it on real data?

Discuss the trade-offs between using hash maps, balanced binary trees, and arrays for data retrieval in high-t

Medium · Data Structures & Algorithms

Discuss the trade-offs between using hash maps, balanced binary trees, and arrays for data retrieval in high-throughput systems.

Approach

1. Restate the input: its shape, its size, and what is guaranteed about it.
2. Name the brute-force solution and its complexity before improving on it.
3. Choose the data structure from the access pattern, not from familiarity.
4. State the target complexity and say which constraint rules the naive version out.

Follow-up

• How does this change if the input no longer fits in memory?
• What is the worst case, and how likely is it on real data?

Explain memory management, stack vs. heap allocation, and how garbage collection impact software performance.

Medium · Data Structures & Algorithms

Approach

1. Say what the runtime actually does before reasoning about the code.
2. Name what is shared across threads and what owns each piece of state.
3. Identify the window where an invariant is briefly untrue.
4. Distinguish a value from a reference to it, and say which one you handed out.

Follow-up

• What happens if two callers reach this at the same time?
• Where could this allocate more than you expect?

Discuss database indexing, query optimization, and when to select SQL versus NoSQL datastores.

Medium · Full-Stack & System Architecture

Approach

1. Name the grain you start from and join outward from it.
2. Check whether any join is one-to-many before aggregating, or the sums inflate.
3. Say which index the query would use, and what makes it unusable.
4. Handle the rows that do not match: that is usually the actual question.

Follow-up

• How does the query change if that join becomes one-to-many?
• What happens to this when the table is ten times larger?

Keep soft-deleted accounts from blocking re-registration

Easy · soft delete · unique constraints · engine differences

app_user holds user_id, tenant_id, email CITEXT, password_hash (NULL for SSO principals), email_verified_at, auth_version, status ('invited','active','suspended','deactivated'), created_at, updated_at, deleted_at. Two live accounts for one address inside a tenant must be impossible, but an address freed by a soft delete must be reusable, and the same tenant may delete and re-register it repeatedly. Write the uniqueness DDL for PostgreSQL 16, then the equivalent for MySQL 8 where partial indexes do not exist, and say what each permits once three deleted rows already hold that address.

Approach

1. Start from what is actually unique: not (tenant_id, email), but (tenant_id, email) among live rows. PostgreSQL says that directly — CREATE UNIQUE INDEX app_user_live_email ON app_user (tenant_id, email) WHERE deleted_at IS NULL. A full constraint over the same two columns burns the address permanently the first time someone deletes an account.
2. Keep case-insensitivity in the type or the index, never in the application: CITEXT as given, or UNIQUE (tenant_id, lower(email)) as an expression index where the extension is unavailable. A case-sensitive unique column is exactly how two accounts for one human appear.
3. For MySQL 8 the predicate has to move inside the key: add a discriminator column that is a constant 0 while the row is live and is set to user_id on delete, with UNIQUE (tenant_id, email, deleted_marker). Live rows share the constant and still collide; deleted rows differ from each other and stop colliding.
4. State the NULL variant and its dependency: leaving the marker NULL for deleted rows also works, because a unique index treats NULLs as distinct — true in MySQL, and true in PostgreSQL only under the default NULLS DISTINCT, which PostgreSQL 15 lets you reverse. Check the polarity against the three existing deleted rows: constant-on-live is what preserves the collision you want, and reversing it silently admits duplicate live accounts.

Follow-up

• A deleted account re-registers with the same address the next day. Do the old resource rows follow the new user_id, and how does the API keep the two principals apart?
• How do you honour an erasure request while resource_revision.actor_user_id still references this table?

Explain the architecture of a RESTful API and how you enforce security, rate limiting, and input validation.

Medium · Full-Stack & System Architecture

Approach

1. Say who the caller is and what they do when the call fails halfway.
2. Define the identity of a request so a retry cannot double-apply it.
3. Separate accepted, pending, failed and confirmed; they are different facts.
4. Design the error taxonomy before the success shape; callers branch on it.

Follow-up

• What happens if the caller retries after a timeout?
• How does a client discover it is on an old version of this contract?

How do microservices communicate efficiently, and what strategies do you use for service discovery and message

Medium · Full-Stack & System Architecture

How do microservices communicate efficiently, and what strategies do you use for service discovery and message queuing?

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

Describe how state management works in modern web applications using framework platforms like React.

Medium · Full-Stack & System Architecture

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

How do you approach designing a scalable system for processing real-time telemetry data from automated laborat

Medium · Full-Stack & System Architecture

How do you approach designing a scalable system for processing real-time telemetry data from automated laboratory equipment?

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

Walk me through the architecture of a major software project listed on your resume, detailing your specific te

Medium · Resume & Domain Deep-Dives

Walk me through the architecture of a major software project listed on your resume, detailing your specific technical contributions.

Approach

1. Fix the scope first: who calls this, how often, and what they do when it fails.
2. Name the read and write paths separately; they rarely have the same bottleneck.
3. Choose a partition key and say what query it makes expensive.
4. State the consistency you need, and where you are willing to be stale.

Follow-up

• What breaks first when traffic grows ten times?
• How does this behave when that dependency is down for an hour?

How do you ensure code quality, maintainability, and test coverage throughout the software development lifecyc

Medium · Resume & Domain Deep-Dives

How do you ensure code quality, maintainability, and test coverage throughout the software development lifecycle?

Approach

1. Clarify what is being asked and what a complete answer contains.
2. State your assumptions explicitly before working the problem.
3. Say what you would check first and why it is the highest-information step.
4. Work from the requirement backwards to the design.

Follow-up

• What assumption would you test first?
• How would you know your answer was wrong?

Explain how your past experience in software development translates into supporting complex scientific or oper

Medium · Resume & Domain Deep-Dives

Explain how your past experience in software development translates into supporting complex scientific or operational workflows.

Approach

1. Clarify what is being asked and what a complete answer contains.
2. State your assumptions explicitly before working the problem.
3. Say what you would check first and why it is the highest-information step.
4. Work from the requirement backwards to the design.

Follow-up

• What assumption would you test first?
• How would you know your answer was wrong?

What methodologies do you use to diagnose, debug, and resolve critical production defects in modern software s

Medium · Resume & Domain Deep-Dives

What methodologies do you use to diagnose, debug, and resolve critical production defects in modern software stacks?

Approach

1. Establish what changed and when, before forming any theory.
2. Pick a bisection that eliminates candidates whichever way it turns out.
3. Check the instrumentation before believing the symptom.
4. Separate the trigger from the cause; the deploy is rarely the bug.

Follow-up

• What would you look at first, and what would it rule out?
• How would you tell a cause from a coincidence here?

Suggested practice checklist

Built from the rounds and topics Eli Lilly and candidates report.

Day 1 — Map the Eli Lilly and loop

• Write out the reported sequence: Application Review, Recruiter Screen, Online Technical Assessment, Multi-Round Interviews, Technical Evaluations, Behavioral Interviews.
• For each round, write one sentence on what it is judging, from the description above, and mark the one you are least ready for.

Deliverable: A one-page map of the 6 reported rounds, with the weakest marked.

Day 2 — Work Behavioral Interviewing

• Spend the session on Behavioral Interviewing, which Eli Lilly and candidates report being tested on.
• Write one worked example in Behavioral Interviewing and time yourself on it.

Deliverable: One timed worked example in Behavioral Interviewing.

Day 3 — Work STAR Method (Situation-Task-Action-Result)

• Spend the session on STAR Method (Situation-Task-Action-Result), which Eli Lilly and candidates report being tested on.
• Write one worked example in STAR Method (Situation-Task-Action-Result) and time yourself on it.

Deliverable: One timed worked example in STAR Method (Situation-Task-Action-Result).

Day 4 — Work Data Structures & Algorithms (DSA)

• Spend the session on Data Structures & Algorithms (DSA), which Eli Lilly and candidates report being tested on.
• Write one worked example in Data Structures & Algorithms (DSA) and time yourself on it.

Deliverable: One timed worked example in Data Structures & Algorithms (DSA).

Day 5 — Answer out loud: Behavioral & Cultural Fit

• Answer aloud, timed: Why do you want to work for Eli Lilly and, and how do your personal ethics align with our company values?
• Answer aloud, timed: Describe a situation where you had to collaborate with a difficult team member or resolve a conflict on a project.

Deliverable: Spoken answers to 2 reported Behavioral & Cultural Fit question(s), under time.

Day 6 — Answer out loud: Data Structures & Algorithms

• Answer aloud, timed: Given an array of integers, write an efficient algorithm to find all unique pairs that sum up to a target value.
• Answer aloud, timed: Explain how you would optimize a solution from $O(N^2)$ to $O(N \log N)$ or $O(N)$ time complexity.

Deliverable: Spoken answers to 2 reported Data Structures & Algorithms question(s), under time.

Day 7 — Answer out loud: Full-Stack & System Architecture

• Answer aloud, timed: Explain the architecture of a RESTful API and how you enforce security, rate limiting, and input validation.
• Answer aloud, timed: How do microservices communicate efficiently, and what strategies do you use for service discovery and message queuing?

Deliverable: Spoken answers to 2 reported Full-Stack & System Architecture question(s), under time.

Behavioural preparation

Behavioural rounds judge the decision you made and what it cost.

Why do you want to work for Eli Lilly and, and how do your personal ethics align with our company values?

Medium · Behavioral & Cultural Fit

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

Describe a situation where you had to collaborate with a difficult team member or resolve a conflict on a proj

Medium · Behavioral & Cultural Fit

Describe a situation where you had to collaborate with a difficult team member or resolve a conflict on a project.

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

Tell me about a time you went above and beyond your standard responsibilities to deliver a project or impress

Medium · Behavioral & Cultural Fit

Tell me about a time you went above and beyond your standard responsibilities to deliver a project or impress colleagues.

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

How do you prioritize competing deadlines when managing multiple technical deliverables?

Medium · Behavioral & Cultural Fit

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?

Give an example of a technical challenge you faced in a past role, how you approached it, and what you learned

Medium · Behavioral & Cultural Fit

Give an example of a technical challenge you faced in a past role, how you approached it, and what you learned from the outcome.

Approach

1. Pick a story where you made the decision, not one where you watched it.
2. State the situation in two sentences and spend the rest on the reasoning.
3. Give the blast radius: what could have broken, and what you measured.
4. Name the disagreement and how you resolved it with evidence.

Follow-up

• What would you do differently if you ran that again?
• How did you know your change caused the improvement?