The purpose of this paper is to propose a new approach to research on gender differences in science that uses the work on expertise in science as a framework for understanding gender differences. Because gender differences in achievement and participation in the sciences are largest in physics, the focus of this review is on physics. The nature of expertise is first discussed and a framework that focuses on factors that influence the emergence of expertise in physics is presented. This is used to interpret what is known about gender differences in science, particularly physics. Next, the potential contributions of the research on gender differences to our understanding of expertise are discussed. Using what is learned from these two areas of research, recommendations are made for future research examining gender differences in physics. It is suggested that such an approach be used for other areas of science, such as chemistry, where large gender differences in achievement and participation also exist.