Molecular psychology is the study of behavior and its underlying brain systems using the tools of molecular biology. Although a variety of biological tools have been used to discover the relation of brain and behavior since the beginning of psychology, these have largely been confined to nonhuman animal models or brain-damaged patients. Technical advances in the past two decades in molecular genetics and in noninvasive brain imaging have transformed the field. These advances made it possible to conduct detailed examinations—across all species and subjects including healthy human volunteers—of the relation among genes, brains, and behavior; gene-by-environment (G×E) interactions; and the underlying regulatory mechanisms of gene expression through life experience and other environmental variables. This chapter provides a brief history of the major milestones of this field, along with updates from the contemporary literature, and introduces the reader to the contents of this volume.