Professional development is defined as the main route to quality teaching, which in turn leads to improved learning for all students. Because the real challenges of professional development become apparent only after the implementation process has begun, the present paper focuses on integrating professional development experiences with classroom practices. The challenges of implementation increase as the student population becomes more diversified due to demographic changes. With Vygotsky's views of development as theoretical background, six principles of effective professional development implementation are outlined for teachers working in classrooms that are inhabited by culturally and linguistically diverse students. These six principles focus on the mediational nature of human development and learning.