The task of validating a teacher assessment and improvement system is similar whether the system operates in the United States or in another country. Chile has a national teacher evaluation system (NTES) that is standards based, uses multiple instruments, and is intended to serve both formative and summative purposes. For the past 6 years the authors have performed validation research on NTES using a variety of methods and data sources. This article describes our validation research agenda, the results of major validation studies, and an integration of the existing evidence, and it offers the authors' preliminary judgment about NTES's validity. The article also offers a critical reflection regarding the decisions taken while driving the long and winding validation road, and the lessons we learned during this politically and methodologically complex journey. (Contains 8 tables, 1 figure, and 5 footnotes.)