In this chapter, we initially provide a short overview of educational effectiveness research and concentrate on the main theories of educational effectiveness developed during the last four decades. Specifically, we present the three main approaches to educational effectiveness, discussing their strengths and limitations by taking into account findings of studies that sought to identify the educational effectiveness factors associated with each of these approaches. The importance of establishing integrated and multilevel models is acknowledged, and a brief description of the main integrated models developed in the 1990s is provided. Finally, we argue that studies on the main approaches to modelling educational effectiveness, as well as those testing the validity of integrated models, were influential not only in establishing the theoretical framework of educational effectiveness research, but also in developing educational effectiveness policies in various countries around the world. Their contribution in understanding the complex nature of educational effectiveness is acknowledged. It is, however, conceded that these theories are not in a position to establish links between research on educational effectiveness and school improvement efforts.