This study describes an evaluation of a theory-based trajectory for professional development called FeTiP (Feedback-Theory into Practice) that aims to have an observable effect on teacher classroom behavior. FeTiP is a multicomponent trajectory for professional development and combines several types of interventions. Its goal is to help teachers expand their feedback behavior in the classroom to provide more, and more effective (i.e. learning-enhancing), feedback. We first describe the foundation of FeTiP, with a central focus on how classroom behavior can be influenced by a multicomponent trajectory of professional development, as this is often a major aim in initiatives for the professional development of teachers but is the most difficult to establish. We describe the effects of FeTiP on the feedback behavior of teachers and attempt to explain why these effects occurred.