This paper examines the historical roots of critical realism in western thought, highlights the dramatic nature of the shift in thought that the radical constructivists are seeking, and critically considers the relevance of radical constructivism in science teacher education. Radical constructivism is an epistemological philosophy that divorces knowing from any notion that reality is the referent of knowledge. Radical constructivists argue that adopting this view, rather than the realist view, will help teachers improve science instruction. The argument was put forward by Ernst von Glasersfeld at the 1990 annual meeting of the National Association for Research in Science Teaching. However, Glasersfeld's position implies a dramatic shift away from critical realism, which has deep historical roots in western thought and which was critical to the development of modern science. Furthermore, the possible effect of radical constructivism on science teaching must be questioned. Many would argue that there is little reason to think that ontological beliefs are more critical in teacher behavior than social and material factors. Nevertheless, ontological belief is an interesting aspect of culture and could be incorporated into a general discussion of cultural issues during a program of science teacher education. The thorough discussion of ontological issues, however, is probably best left for graduate education. (Author/TJH)