The Diagnostic Teacher Assessment in Mathematics and Science (DTAMS) was developed to measure the content knowledge and pedagogical content knowledge of middle-school teachers. Its reliability and validity were initially established by reviewing national standards for content and use of expert question writing teams and reviewers. DTAMS was administered to approximately 1,600 middle-school mathematics teachers in 17 states. Subsequent analyses using structural equation modeling and item response theory were performed as part of a multistage validation process. This evaluation contributes to the body of work describing the reliability and validity of these assessments. The results of this study confirm trends in middle-school mathematics teacher preparation and certification and help explain middle-school student mathematics achievement levels. (Contains 5 tables and 3 figures.)