We examine three trends in the assessment of teaching competencies, focusing on implications for validity arguments around those assessments. One important trend is a shift from measuring only teachers’ content knowledge of mathematics to additionally measuring their mathematical knowledge for teaching. A second trend is the move toward measuring practical competencies through performance measures. A third trend represents a shift in perspective about the purpose of teacher assessment, pointing toward formative assessment of teaching competency to inform teacher learning. We highlight in these trends three different types of validity considerations: shifts in construct definition and in approaches to defining the construct, considerations around construct relevance and operationalization in new testing formats, and considerations of use and interpretation.