The purpose of this chapter is to illustrate the application of the model on differentiated teacher effectiveness in a specific research project. The project was conducted to develop a theoretical framework of educational effectiveness research based on Creemers' (1994) comprehensive model of educational effectiveness. Creemers' model and the main findings of the four studies conducted to test its validity (De Jong and Westerhof 1998, Reezigt et al. 1999, Driessen and Sleegers 2000, Kyriakides et al. 2000) have been presented in Chapter 8 and referred to elsewhere in the book. Six issues arising from these studies influenced the design of the study.