# Principal Software Engineer
**Role:** Software Engineer · **Level:** L7
**Company:** [Criteo](https://scaleengineer.com/companies/criteo)
**Difficulty:** Hard
**Salary:** US$170000 - US$220000
**Experience:** 8 - 15
**Timeline:** ~14 days
The Principal Software Engineer (L7) interview at Criteo is a rigorous process designed to assess deep technical expertise, leadership potential, and strategic thinking. Candidates are expected to demonstrate a strong command of software engineering principles, experience in designing and scaling complex systems, and the ability to mentor and guide other engineers. The interview process typically involves multiple rounds, including technical deep dives, system design, behavioral assessments, and a final discussion with senior leadership.
Canonical: https://scaleengineer.com/interviews/criteo/l7-software-engineer
---
## Overall evaluation

- Technical Proficiency
- Leadership and Impact
- Behavioral and Cultural Fit

## Questions asked

- Design a distributed caching system for a high-traffic website.
- How would you optimize the performance of a slow-running API?
- Describe a time you had to influence a team's technical direction.
- What are the trade-offs between SQL and NoSQL databases for a specific use case?
- How do you approach mentoring junior engineers?
- Tell me about a complex technical problem you solved.
- How would you design a system to detect fraudulent ad clicks?
- What are your thoughts on microservices vs. monolith architectures?
- Describe your experience with cloud platforms like AWS, Azure, or GCP.
- How do you handle technical debt?
- What is your approach to code reviews?
- How do you stay up-to-date with new technologies?

## Preparation tips

### lists

- Deeply understand Criteo's business, products, and technology stack.
- Review fundamental computer science concepts (data structures, algorithms, operating systems, networking).
- Practice system design problems, focusing on scalability, reliability, and trade-offs.
- Prepare specific examples from your past experience that demonstrate leadership, problem-solving, and impact.
- Familiarize yourself with common ad-tech concepts and challenges.
- Practice articulating your thought process clearly and concisely.
- Research the interviewers on LinkedIn to understand their backgrounds.

### studyPlan

- {"title":"Data Structures & Algorithms","longDescription":"Weeks 1-2: Focus on core data structures and algorithms. Review common patterns and complexity analysis. Practice problems on platforms like LeetCode (Medium/Hard).","shortDescription":"Weeks 1-2: DSA fundamentals. LeetCode (Medium/Hard)."}
- {"title":"System Design","longDescription":"Weeks 3-4: Dive into system design principles. Study topics like distributed systems, databases, caching, load balancing, and message queues. Read system design case studies.","shortDescription":"Weeks 3-4: System Design. Distributed systems, databases, caching."}
- {"title":"Behavioral Preparation","longDescription":"Week 5: Prepare for behavioral questions using the STAR method (Situation, Task, Action, Result). Reflect on your career experiences, focusing on leadership, challenges, and successes.","shortDescription":"Week 5: Behavioral preparation. STAR method."}
- {"title":"Company and Domain Research","longDescription":"Week 6: Research Criteo's ad-tech domain, recent news, and technical challenges. Understand the ad ecosystem and Criteo's role within it. Prepare questions for the interviewers.","shortDescription":"Week 6: Criteo research. Ad-tech domain."}

## Location differences

- {"location":"Paris","differences":{"tips":["Be prepared to discuss your contributions to open-source projects.","Highlight experience with high-traffic, low-latency systems.","Showcase your ability to influence technical direction and mentor junior engineers."],"interviewFocus":["Emphasis on distributed systems design and scalability.","Assessment of leadership and mentorship capabilities.","Evaluation of strategic thinking and long-term technical vision."],"commonQuestions":["How would you design a real-time bidding system for online advertising?","Discuss a time you had to make a significant technical trade-off. What was the outcome?","How do you approach performance optimization in a large-scale distributed system?","Describe your experience with cloud-native architectures (e.g., Kubernetes, microservices)."]}}
- {"location":"New York","differences":{"tips":["Prepare examples demonstrating your ability to break down complex problems.","Emphasize your experience in agile development environments.","Be ready to discuss your understanding of Criteo's business and the ad-tech landscape."],"interviewFocus":["Focus on problem-solving and analytical skills.","Assessment of collaboration and communication within a team.","Evaluation of adaptability to new technologies and methodologies."],"commonQuestions":["Design a system to handle millions of concurrent users for a social media platform.","Tell me about a time you led a team through a challenging technical project.","How do you ensure the reliability and fault tolerance of a distributed system?","What are your thoughts on the future of AI in ad-tech?"]}}
- {"location":"London","differences":{"tips":["Quantify your achievements with data and metrics.","Be prepared to discuss your contributions to system architecture and design.","Showcase your ability to mentor and grow engineering talent."],"interviewFocus":["Emphasis on data-driven decision making and impact.","Assessment of technical depth in specific domains (e.g., machine learning, data engineering).","Evaluation of ability to drive technical initiatives from conception to delivery."],"commonQuestions":["How would you design a recommendation engine for an e-commerce platform?","Describe a situation where you had to deal with a major production incident. How did you resolve it?","What are the key considerations when designing for massive data volumes?","Discuss your experience with A/B testing and experimentation frameworks."]}}

## Round 1: Data Structures and Algorithms
**Type:** Technical Coding Interview · **Difficulty:** Hard · **Duration:** 60 min
Assess core coding skills and problem-solving using data structures and algorithms.
This round focuses on your core programming skills and problem-solving abilities. You will be given one or two coding problems, typically involving data structures and algorithms. The interviewer will assess your ability to understand the problem, devise an efficient solution, write clean and correct code, and analyze its time and space complexity. Expect to discuss trade-offs and edge cases.
**Interviewers look for:** Strong coding skills.; Ability to translate requirements into working code.; Understanding of fundamental data structures and algorithms.; Clean and maintainable code.
**Evaluation criteria:** Correctness of code; Efficiency of algorithms; Code quality and readability; Problem-solving approach; Understanding of time and space complexity
**Common rejection reasons:** Lack of fundamental CS knowledge.; Inability to articulate solutions clearly.; Poor performance on coding challenges.; Not meeting the bar for algorithmic thinking.
## Questions

- Given a binary tree, find the lowest common ancestor of two given nodes.
- Implement a function to find the k-th largest element in an unsorted array.
- Design a data structure that supports insert, delete, search, and getRandom in O(1) average time.

## Preparation tips

- Practice coding problems on platforms like LeetCode, HackerRank, or Coderbyte.
- Focus on common data structures (arrays, linked lists, trees, graphs, hash maps) and algorithms (sorting, searching, dynamic programming, graph traversal).
- Practice explaining your thought process out loud as you code.
- Be prepared to discuss edge cases and optimize your solution.

## Round 2: System Design and Architecture
**Type:** System Design Interview · **Difficulty:** Hard · **Duration:** 60 min
Assess ability to design scalable and reliable distributed systems.
This round evaluates your ability to design and architect large-scale distributed systems. You'll be presented with a high-level problem (e.g., design Twitter's feed, design a URL shortener) and expected to break it down, identify components, define APIs, and discuss scalability, reliability, and trade-offs. Focus on clarifying requirements, making reasonable assumptions, and justifying your design choices.
**Interviewers look for:** Ability to design complex, scalable, and reliable systems.; Deep understanding of distributed systems principles.; Proficiency in choosing appropriate technologies and justifying decisions.; Consideration of non-functional requirements (scalability, availability, latency, consistency).
**Evaluation criteria:** System design approach; Scalability and performance; Reliability and fault tolerance; Choice of technologies and justifications; Understanding of trade-offs; Handling of edge cases and failure scenarios
**Common rejection reasons:** Inability to design scalable and reliable systems.; Lack of consideration for trade-offs.; Poor understanding of distributed systems concepts.; Not addressing non-functional requirements adequately.
## Questions

- Design a system to count unique visitors to a website in real-time.
- Design a distributed rate limiter.
- How would you design a notification system for millions of users?

## Preparation tips

- Study common system design patterns and architectures.
- Understand concepts like load balancing, caching, databases (SQL vs. NoSQL), message queues, CDNs, and microservices.
- Practice designing systems for scale, considering factors like read/write patterns, data partitioning, and consistency models.
- Be prepared to discuss trade-offs between different design choices.
- Review case studies of large-scale systems.

## Round 3: Behavioral and Leadership Assessment
**Type:** Behavioral and Managerial Interview · **Difficulty:** Medium · **Duration:** 45 min
Assess leadership, teamwork, and cultural fit through behavioral questions.
This round focuses on your past experiences, leadership potential, and how you work within a team. You'll be asked behavioral questions designed to elicit specific examples of your accomplishments, challenges faced, and how you handled them. Prepare to discuss your career goals, motivations, and why you're interested in Criteo and this specific role.
**Interviewers look for:** Evidence of leadership and initiative.; Ability to work effectively in a team.; Strong communication skills.; Alignment with Criteo's culture and values.; Passion for technology and the company's mission.
**Evaluation criteria:** Leadership qualities; Teamwork and collaboration; Problem-solving approach in past projects; Communication skills; Cultural fit; Motivation and career aspirations
**Common rejection reasons:** Lack of leadership experience.; Poor communication or collaboration skills.; Inability to provide specific examples of impact.; Not demonstrating alignment with Criteo's values.
## Questions

- Tell me about a time you had to lead a project from start to finish.
- Describe a situation where you disagreed with a teammate or manager. How did you handle it?
- How do you prioritize your work when faced with multiple competing tasks?

## Preparation tips

- Prepare specific examples using the STAR method (Situation, Task, Action, Result) for common behavioral questions.
- Think about situations where you demonstrated leadership, teamwork, conflict resolution, and problem-solving.
- Research Criteo's company culture and values.
- Be ready to articulate your strengths and weaknesses.
- Prepare thoughtful questions to ask the interviewer about the team, role, and company.

## Round 4: Executive and Strategic Alignment
**Type:** Senior Leadership / Executive Interview · **Difficulty:** Hard · **Duration:** 60 min
Discuss strategic thinking, technical vision, and leadership with senior management.
This final round is typically with a senior leader (Director or VP). It's a broader discussion about your career, your technical philosophy, your leadership style, and your vision for the future. They will assess your strategic thinking, your ability to influence at a high level, and your potential to contribute to Criteo's long-term technical success. Be prepared to discuss your biggest technical achievements and challenges.
**Interviewers look for:** A clear technical vision and strategy.; The ability to influence technical direction across teams.; Understanding of how technology drives business value.; Experience mentoring and developing other engineers.; Strong communication and presentation skills.
**Evaluation criteria:** Strategic thinking; Technical vision; Business acumen; Influence and impact; Mentorship capabilities; Communication with senior leadership
**Common rejection reasons:** Lack of strategic vision.; Inability to connect technical decisions to business impact.; Poor communication with senior stakeholders.; Not demonstrating the gravitas expected of a Principal Engineer.
## Questions

- What is your vision for the future of ad-tech engineering?
- Describe a time you had to make a significant technical decision with incomplete information.
- How would you foster innovation within an engineering team?

## Preparation tips

- Think about your long-term career goals and how this role aligns with them.
- Prepare to discuss your technical philosophy and approach to engineering leadership.
- Consider how you would contribute to Criteo's technical strategy.
- Be ready to discuss your experience mentoring and growing engineering teams.
- Have thoughtful questions prepared about the company's direction and challenges.