- A founder of the Extreme Programming, along with Kent Beck and Ward Cunningham.
- Creator of Test Driven Development.
- An original signatory of The Manifesto for Agile Software Development.
- A founding Member of Hillside Group with Jim Coplien, Grady Booch, Ward Cunningham, Ralph Johnson, Ken Auer and Hal Hildebrand.
- Wrote the SUnit unit testing framework for Smalltalk, which spawned the xUnit series of frameworks, notably JUnit for Java.
- Proposed CRC cards with Ward Cunningham.
- Began experimenting with the idea of applying patterns to programming with Ward Cunningham
- Optimism is an occupational hazard of programming; feedback is the treatment.
- Make it work, make it right, make it fast.
- At some point software design becomes less about what and more about when.