For over a decade, professional learning communities (PLCs) have been touted as an effective way to build upon the knowledge and skills of experienced teachers, yet much of the evidence base is derived from self-reports by practitioners. Although several generations of school reform (the standards movement, No Child Left Behind, and now the Common Core State Standards) have cited improving teacher effectiveness as key to improving student achievement, little change has occurred in the nature of professional development. This article argues that professional development generally, and PLCs in particular, would benefit from the insights gleaned from the extensive literature on teacher expertise that focuses on how well teachers understand the content they teach and how well they understand how students learn that content. (Contains 1 note.)