Research trends on the required professional knowledge for university-based teacher educators in Europe and the US under the ongoing educational reform since the 80s were explored. In the early 90s, it was indicated that the 80s’ educational policies generated some momentum toward an authoritative, research-driven vision of teaching practice and this review showed that this momentum accelerated until the research outcomes were required for administrative control of teacher educators as well as teachers. Under this situation, three research streams for teachers’ professional knowledge emerged, which provide references for teacher educators as well, i.e., a knowledge-based model based on psychology, a reflective practice model based on a concept of reflection, and self-study, which is a blended model of the former two models. Although new conceptions of professional knowledge as alternatives to traditional knowledge have been proposed in these three streams, when we look through them carefully, the difficulty of finding agreement on the definition of professional knowledge is clear. Professional knowledge is a usable concept for today’s standardized educational atmosphere; therefore, the concept of professional knowledge for teaching, teachers, and teacher educators should be examined critically.