Assessments of teacher content knowledge are increasingly designed to provide evidence of the content knowledge needed to carry out the moment-to-moment work of teaching. Often these assessments focus on content knowledge only used in teaching with the goal of testing types of professional content knowledge. In this paper, we argue that while this general approach has produced powerful exemplars of new types of assessment tasks, it has been less successful in developing tests that provide more general evidence of the range of content knowledge associated with particular teaching practices. To illustrate a more systematic approach, we describe the use of evidence-centered design (ECD) to develop an assessment of content knowledge for teaching (CKT) in the area of secondary physics-energy.