This study was conducted in order to examine the process of teacher development in a close interplay between educational principles and classrooms in a Singapore practicum curriculum. The process of teacher development is regarded as a continuous, self-renewing, lifelong process. Since no comprehensive constructivist model of teacher development has been advanced in the literature to guide in the design of teacher education programs, this paper seeks to spell out the rationale and relationship of the four basic components of a teacher development program with respect to: (1) subject matter; (2) teaching methods; (3) educational principles; and (4) classroom practice. Humanistic constructivism provides the underlying philosophy and pedagogy of the proposed teacher development model. Through a better articulation of teacher roles, internalization of educational principles, and reflection of classroom practices, teachers' professional development grows. Based on this model of teacher development in a practicum curriculum, some research evidence of professional growth is presented using student teachers' self-appraisal of their own and their cooperating teachers' classroom practice. (Author/LL)