This study focuses on improving teacher feedback during active learning. Changing teachers' behavior sustainably, however, is very difficult. Several conditions should be taken into account, and programs should build on teachers' cognitions and practices. Effects of a specifically designed professional development program on 16 elementary schoolteachers' knowledge, beliefs, perceived problems, and classroom behavior were examined via observations, a beliefs instrument, and a questionnaire prior to and twice after the program was implemented. Results show that several aspects of feedback during active learning were improved, both in the short and in the long term. It is concluded that the professional development of teachers can be effective and sustainable, if certain conditions are met.