This chapter addresses the issue of teacher education and development from various perspectives. Several categories of mathematics educators are characterized and their development and roles in the teaching/learning processes are summarized. Cooperation between teachers and researchers as well as the concept of teachers as researchers are discussed from different points of view. The crucial role that observations play at all levels is analyzed and illustrated by two different models of implementation of observations into teachers' and researchers' practice. Throughout the chapter the influence of the research of Guy Brousseau on mathematics education research is recognized.