This book examines the relationship between teacher theorizing and teacher action as illustrated by the curricular and instructional practices of teachers. The authors show that all teaching is guided by theory developed by the teachers. Teachers could not begin to practice without some knowledge of the context of their practice and without ideas about what can and should be done in those circumstances. In this sense, teachers are guided by personal, practical theories that structure their activities and guide them in making decisions.This literature is very significant in explaining and interpreting many phenomena of schooling such as why teachers alter curriculum documents and other policies, how inservice education can be improved, how supervisors can help teachers to improve their practices, and how administrators can become leaders to improve education. This perspective has broad and specific implications for every facet of education. Those interested in teacher education and development, in supervision, in curriculum, and in administration will find it especially relevant.