Although the importance of professional development for improved student learning is widely recognized and many studies have been devoted to enhancing its effectiveness, the specific mechanism underlying teacher learning and how it translates into actual classroom practices are not well understood. This paper is an attempt to fill this gap in understanding by grounding professional development models in two theories of human development. Adopting a theoretical stance provides an explanation of the processes underlying teacher development. In addition, theories also allow an understanding of what factors might contribute to effective professional development implementation. The social cognitive theory and the sociocultural theory are chosen based on numerous empirical studies indicating their validity in explaining and predicting human development and change. These theories provide foundations for understanding why professional development leads to teacher development and guidelines for translating teacher learning into improved classroom instruction.