This study explores the effect of a teacher development programme, emphasising the development of critical thinking, in producing changes in teaching practice. The theoretical point of view adopted in this thesis considers the active role played by learners in the construction of knowledge and Freire's critical pedagogy. Freire's proposition is that education is a political act and his epistemology and consequent pedagogy is based on challenges: those challenges in which learners, as members of a society, are presently immersed and from which they must emerge to promote changes for a more democratic world. The research I have carried out is a case-study approach to an exploration of an in-service course, in Brazil, during which eleven secondary science and mathematics teachers discussed their views and had contact with the constructivist approach and the methodology of action research. During this period the teachers designed critical action-research projects, developing approaches to the construction of knowledge by pupils in their own classroom. Their projects were developed over a four to six week period. The teachers were observed and video taped in their classrooms in the course of three lessons. Finally, each teacher reported their results and their perceptions of the impact of these on their pupils and whether this kind of intervention is a viable way of introducing changes. Sixteen months later, they were encouraged towards self-critical reflection on the whole process in an extended interview. In this study, the answers are intended to contribute to an understanding of the changes needed in teacher development programmes, as well as to reveal some obstacles to the introduction of innovations in classroom practice. This research vindicates a Freirean perspective on teachers critical constructivist reflection as a mechanism for promoting democratic classroom change. The implications of these teachers' collaborative critical action research extends well beyond the immediate context into a broad educational arena.