Pair Programming: All production code written by two people at one keyboard. Test-Driven Development (TDD): Write the test before the code. Refactoring: Continuously improve design without changing behavior. Continuous Integration: Integrate and test multiple times per day. Small Releases: Deploy to production frequently, not quarterly. Planning Game: Customer writes stories; developers estimate; customer prioritizes. On-Site Customer: A real customer is available to answer questions daily. Collective Code Ownership: Anyone can change any part of the codebase. Simple Design: The simplest design that passes all tests. Coding Standards: Shared conventions so any developer can work on any code. 40-Hour Week: Overtime indicates a planning problem, not a work ethic problem. System Metaphor: A shared story describing how the system works.