Test-driven development (TDD) has gained recent attention with the popularity of the Extreme Programming agile software development methodology. Advocates of TDD rely primarily on anecdotal evidence with relatively little empirical evidence of the benefits of the practice. This research is the first comprehensive evaluation of how TDD affects software architecture and internal design quality. Formal controlled experiments were conducted in undergraduate and graduate academic courses, in a professional training course, and with in-house professional development projects in a Fortune 500 company. The experiments involved over 230 student and professional programmers working on almost five hundred software projects ranging in size from one hundred to over 30,000 lines of code. The research also included a case study of fifteen software projects developed over five years in a Fortune 500 corporation. This research demonstrates that software developers applying a test-first (TDD)