This paper argues that Educational Effectiveness Research (EER) should establish closer links with research on teacher professional development. A dynamic integrated approach (DIA) to teacher professional development is proposed. The methods and results of a study comparing the impact of the DIA and the Holistic Approach (HA) to teacher professional development are presented. Teaching skills of 130 teachers and achievement of their students (n = 2356) were measured at the beginning and at the end of the intervention. Teachers found to be at a certain developmental stage were randomly allocated evenly into 2 groups. The first group employed the DIA and the second the HA. Teachers employing the DIA managed to improve their teaching skills more than teachers employing the HA. The use of the DIA also had a significant impact on student achievement. Implications of findings for the use of EER for improvement purposes are drawn. (Contains 4 tables.)