Two varieties of constructivism are distinguished. In part 1, the psychological or "radical" constructivism of von Glasersfeld is discussed. Despite its dominant influence in science education, radical constructivism has been controversial, with challenges to its principles and practices. In part 2, social constructivism is discussed in the sociology of scientific knowledge. Social constructivism has not been primarily concerned with education but has the most direct consequences in view of its challenge to the most fundamental, traditional assumptions in the philosophy of science and to the practice of science itself.