The research presented here aimed to explore the notion of PCK in studies which examine primary pre-service and in-service science teachers' Pedagogical Content Knowledge (PCK). The method of critical review was used to achieve this goal. The data came from the purpose of studies, the research questions that are posed and the notion of PCK which is followed in the particular investigations. It was found that the ambiguity of the term of PCK creates problems for research. It is concluded that it needs clarification of the notion of PCK so that it is explicit what is precisely searched.