Science teachers' content knowledge is an important influence on student learning, highlighting an ongoing need for programs, and assessments of those programs, designed to support teacher learning of science. Valid and reliable assessments of teacher science knowledge are needed for direct measurement of this crucial variable. This paper describes multiple sources of validity and reliability (Cronbach's alpha greater than 0.8) evidence for physical, life, and earth/space science assessments--part of the Diagnostic Teacher Assessments of Mathematics and Science (DTAMS) project. Validity was strengthened by systematic synthesis of relevant documents, extensive use of external reviewers, and field tests with 900 teachers during assessment development process. Subsequent results from 4,400 teachers, analyzed with Rasch IRT modeling techniques, offer construct and concurrent validity evidence.