This chapter explains a Popperian approach to build teacher knowledge. It discusses the need for schools to use a critical approach for knowledge development and knowledge sharing. The chapter then discusses a Popper's three core concepts of critical rationalism is presented for its relevance in teachers' knowledge development, followed by an illustration of how Popper's schema promotes teachers' critical thinking and knowledge growth. His three core concepts of critical rationalism are fallibilism, criticism, and verisimilitude. Crucial to Popper's concept of critical rationalism is the concept of verisimilitude because it is only the idea of the truth that allows us to speak sensibly of fallibilism and criticism. Fullan argues that schools need to focus on knowledge production and sharing if teaching is to be seen as an intellectual profession that principals need to understand the role of knowledge development in the performance of their schools. The chapter concludes that the critical self-learning does facilitate a useful trajectory for knowledge growth.