During the last three decades, scholars have proposed several conceptual structures to represent teacher knowledge. A common denominator in this work is the assumption that disciplinary knowledge and the knowledge needed for teaching are distinct. However, empirical findings on the distinguishability of these two knowledge components, and their relationship with student outcomes, are mixed. In this replication and extension study, we explore these issues, drawing on evidence from a multi-year study of over 200 fourth- and fifth-grade US teachers. Exploratory and confirmatory factor analyses of these data suggested a single dimension for teacher knowledge. Value-added models predicting student test outcomes on both state tests and a test with cognitively challenging tasks revealed that teacher knowledge positively predicts student achievement gains. We consider the implications of these findings for teacher selection and education.