To understand how subfields of psychology relate to each other as a whole, we analyzed 40 years (from 1979 to 2009) of journal citation data collected from 17 American Psychological Association journals. The results reveal two stable underlying dimensions of psychological knowledge-basic versus applied, and population-specific versus population-general-that organize subfields of psychology. Within the structure, personality and social psychology is located at the heart of psychological knowledge. Analysis of the dynamic flow of knowledge between subfields of psychology further reveals that although the subfields engage in clear division of labor, they also engage in dynamic transactions of knowledge. Finally, an emergent subfield would first obtain its intellectual nutrients from the established disciplines. Once it has found its own niche, it turns into a spin-off and starts to assume the role of knowledge supplier. The implications of these results for psychology as a science are discussed.