This study presents an evaluation of a theory-based trajectory for professional development called FeTiP (FeedbackTheory into Practice). It aims to have an observable effect on teacher classroom behavior. The authors describe the effects of FeTiP on the feedback behavior of teachers and attempt to explain why these effects occurred. The findings reveal that teachers showed significant progress in the frequency of the feedback they provided after following FeTiP. In the post-tests, they also provided significantly more specific feedback, and their ratio of positive and negative feedback increased. The authors found no differences for age, gender, or experience in the total frequency of feedback, specific feedback, and the ratio of positive and negative feedback at the pre-test condition.