When a new teacher is learning to teach, what is she or he learning? When an experienced teacher ‘knows’ how to teach, what is that teacher smart about? Despite a growing literature on teacher education, the first of these questions, which centers on the ‘learning’ in the learning to teach process, has often been evaded. Studies of learning to teach have focused on the socialization of teachers (for example, Zeichner, 1987; Zeichner and Tabachnick, 1985), the impact of particular programs (Feiman-Nemser, 1983), the effect of cooperating teacher and university supervisors (Richardson-Koehler, 1988; Zimpher, 1987), and similar topics, but few studies have directly addressed what is learned. And despite a substantial data base on teacher effectiveness, knowledge about what teachers know has been elusive.