Kent Beck created Extreme Programming (XP) in the late 1990s, developing a set of software engineering practices — test-driven development (TDD), pair programming, continuous integration, small releases, refactoring — that proved to be, in the Poppendiecks' framing, lean principles expressed in software development terms before lean software had a name.
XP practices as lean practices
mary-poppendieck and tom-poppendieck's first book, lean-software-development-agile-toolkit-2003, explicitly positioned lean principles as complementary to XP and Scrum rather than competing with them. The reasoning was principled, not diplomatic: XP's practices instantiate lean principles at the team level. Test-driven development is build-integrity-in — quality is verified at the moment of creation, not inspected after the fact. Continuous integration is a form of eliminate-waste (eliminating the waste of integration problems that accumulate when integration is deferred) and amplify-learning (surfacing feedback immediately). Small releases instantiate deliver-as-fast-as-possible and reduce inventory waste from the seven-wastes-of-software.
Relationship between lean and XP communities
Beck was a signatory of the Agile Manifesto (2001) and is identified primarily with the Agile community, not the lean community. The Poppendiecks bridged these communities by providing lean manufacturing foundations for practices that Agilists had developed from first principles in software. This gave XP practitioners a theoretical grounding — why these practices work at a systems level — while giving lean practitioners concrete implementations.
Set-based design contrast
Beck's XP approach tends toward iterative, commitment-based design: build something, get feedback, revise. This contrasts with set-based-design, the lean approach the Poppendiecks adopted from taiichi-ohno's engineering practice: keep multiple design options open until the last-responsible-moment. The Poppendiecks treated these as complementary rather than conflicting: set-based thinking applies to architectural and design decisions with long-horizon consequences; XP iteration applies to implementation.