By examining how people learn, the educational theories of Dewey, Piaget, Vygotsky and Bruner can be synthesized to give this set of core Constructivist principles. Principles of effective mathematics teaching: (1) allows learning that is "active" and "reflective". Students are required to transfer key concepts to new situations; (2) allows learning that is continuous. It is designed to allow students to understand new experiences in the light of "prior knowledge"; and (3) allows learning that is social. It requires "interaction" to develop a deep conceptual understanding, positive relationships and a classroom community. In this article, the authors examine a case study of Ms. Parker, a 7th grade middle school mathematics teacher. They consider her introductory lesson on surface area, in the light of these Constructivist beliefs.