Constructivism is one of the most popular and fastest growing paradigm in recent pedagogy. The fundamental assumption of constructivism is the thesis that knowledge is always and with necessity human construction. All knowledge about the world around us is the result of our experience. Of course, there are many forms of constructivism, although the Polish educational literature generally mentions two faces of constructivism: the personal (inspired by Piaget’s psychology) and the social (based on the psychology of Vygotski). It appears that radical constructivism has not been given proper discussion in Polish pedagogy. This theory, what is important, is the most famous and important kind of constructivism because of its epistemological and educational assumptions and consequences. Therefore in this article I want to achieve two goals: 1) to present the principal assumptions and the thesis of radical constructivism as articulated by Ernst von Glasersfeld; 2) to discuss selected controversies that this model of knowledge and learning has brought about.