The purposes of this chapter are to illustrate the influential ideas in teacher effectiveness, to argue the case for a differentiated model of teacher effectiveness, and to justify the structure of the book. We draw the ideas from key research studies, but there is no attempt to provide a comprehensive review of the literature, because the field has been systematically reviewed by others (see for example Creemers 1994, 1996, Mortimore 1998, Reynolds et al. 1994, Scheerens 2000, Scheerens and Bosker 1997, Teddlie and Reynolds 2000). The research studies we particularly draw upon are: Coleman et al. 1966, Jencks et al. 1972, Bernstein 1971, Brookover et al. 1979, Brophy and Good 1986, Rutter et al. 1979, Mortimore et al. 1988, Wittrock 1986, Creemers 1994, Mortimore 1998, Teddlie and Stringfield 1993, Reynolds et al. 1994, Teddlie and Reynolds 2000, Scheerens 2000, Wright et al. 1997.