# About Joe Leo | Def Method

> Joe Leo founded Def Method to modernize high-stakes software systems and developed GUIDE as an operating philosophy for AI-assisted engineering.

![Joe Leo](https://www.defmethod.com/assets/podcast/joe-leo-headshot-13a12a4cb17a31cc720de43d726dc347bd1a4a930a25c3f4c23bbf5f7f30a0fa.jpg)

# Joe Leo

Founder & CEO, Def Method

I founded Def Method in 2014 with a simple premise: the most valuable engineering work happens when the stakes are high and getting it wrong isn't an option.

I've helped engineering teams at companies like [Citi](https://www.defmethod.com/case-studies/building-a-derivative-pricing-platform-at-citi), Walmart, and [Paxos](https://www.defmethod.com/case-studies/streamlining-compliance-at-paxos) modernize systems that couldn't afford to break. Revenue-critical platforms. Regulated environments. Operationally sensitive infrastructure.

I'm also the co-author of [**The Well-Grounded Rubyist**](https://www.manning.com/books/the-well-grounded-rubyist-fourth-edition) (now in its 4th edition) and co-host of [**The Ruby AI Podcast**](https://www.defmethod.com/podcast), where we explore the intersection of Ruby and artificial intelligence.

That combination of high-stakes modernization work and practical AI exploration shaped [**GUIDE**](https://www.defmethod.com/guide-principles): our operating philosophy for helping teams preserve understanding, strengthen governance, set direction, and build evidence before implementation accelerates.

[LinkedIn](https://linkedin.com/in/jleo3)

## Speaking & Podcast

I regularly speak at conferences about Ruby, Rails, and engineering leadership. I'm also the co-host of [The Ruby AI Podcast](https://www.defmethod.com/podcast).

[![The Ruby AI Podcast](https://www.defmethod.com/assets/podcast/ruby-ai-podcast-hero-04aa6ef3e79407408f09b7d0b56ec5e1ed6029d83ca09739dd4c92c0ca7cabbf.jpg) ### The Ruby AI Podcast Biweekly conversations exploring Ruby in the age of AI.](https://www.defmethod.com/podcast)

[![Silent Killers: Lessons from the Brink - RailsConf 2025](https://www.defmethod.com/assets/railsconf-2025-thumbnail-71319990ceca0e3b84c82c4ae4663342cfc52584e1cce326a9190092b62e743f.jpg) RailsConf 2025 ### Silent Killers: Lessons from the Brink](https://www.youtube.com/watch?v=OIOU7p4HUQM)

## Latest Insights

[View all →](https://www.defmethod.com/essential-complexity)

[August 04, 2026 ### No More Free Rides Amazon's retail site went down three times in a week. The cause: an AI agent acted on bad guidance from a stale internal wiki. This is what the Understanding principle is for.](https://www.defmethod.com/essential-complexity/no-more-free-rides)
[July 31, 2026 ### Governance Like a Warm Hugging Face An OpenAI model escaped its sandbox during an internal evaluation and breached Hugging Face. What made it a contained incident instead of a catastrophe was governance infrastructure Hugging Face had already built before any of this happened.](https://www.defmethod.com/essential-complexity/governance-like-a-warm-hugging-face)
[July 27, 2026 ### Somebody Has to GUIDE Us AI unbundles implementation from understanding. The person writing the code no longer has to know the system. That's the gain — and that's the gap.](https://www.defmethod.com/essential-complexity/somebody-has-to-guide-us)

![The Well-Grounded Rubyist book cover](https://www.defmethod.com/assets/well-grounded-rubyist-be29b9cece82ea504aa4b2194c22d5c6fbc69f1300e11c4d01b24447a6dae336.webp)

## The Well-Grounded Rubyist

4th Edition — Co-authored with David A. Black

The Well-Grounded Rubyist is a comprehensive guide to Ruby that has helped thousands of developers master the language. Now in its 4th edition, it covers Ruby 3.x and modern Ruby idioms while maintaining the deep, thorough approach that made it a classic.

Whether you're new to Ruby or looking to deepen your understanding, this book provides the foundation you need to write confident, idiomatic Ruby code.

[Buy](https://www.manning.com/books/the-well-grounded-rubyist-fourth-edition?utm_source=defmethod&utm_medium=affiliate&utm_campaign=book_black5&a_aid=defmethod&a_bid=b62bd6b7&chan=mm_linkedin&utm_campaign=22914427-book_black5&utm_source=linkedin)

## Want to work together?

If you're responsible for a high-stakes system that needs to modernize for AI-assisted development without losing engineering confidence, let's talk.

[Start with a GUIDE Assessment](https://www.defmethod.com/assessment)