This paper discusses best practices in teachers’ professional development (PD) in the United States (U.S.). We begin by presenting a conceptual framework for effective professional development, which suggests five key features that make professional development effective—content focus, active learning, coherence, sustained duration, and collective participation. We then describe the findings from recent U.S. research that has tested the five features, with an emphasis on the results of rigorous randomized control trials. We discuss several insights gained from this work and that have helped refine the framework. They are that (a) changing procedural classroom behavior is easier than improving content knowledge or inquiry-oriented instruction techniques; (b) teachers vary in response to the same PD; (c) PD is more successful when it is explicitly linked to classroom lessons; (d) PD research and implementation must allow for urban contexts (e.g., student and teacher mobility); and (e) leadership plays a key role in supporting and encouraging teachers to implement in the classroom the ideas and strategies they learned in the PD. We then examine three major trends in how professional development for teachers is evolving in the U.S.—a move away from short workshops, linking teacher PD to evaluations, and the use of video technology to improve and monitor the effects of PD. Finally, we discuss the challenges faced by districts and schools in implementing effective professional development.