Determining the biological bases for behavior—and the extent to which we can observe and explain their neural underpinnings—requires a bold, broadly defined research methodology. The interdisciplinary entries in this handbook are organized around the principle of “molecular psychology,” which unites cutting-edge research from such wide-ranging disciplines as clinical neuroscience and genetics, psychology, behavioral neuroscience, and neuroethology. For the first time in a single volume, leaders from diverse research areas present their work in which they use molecular approaches to investigate social behavior, psychopathology, emotion, cognition, and stress in healthy volunteers, patient populations, and an array of nonhuman species including nonhuman primates, rodents, insects, and fish. Chapters draw on molecular methods covering candidate genes, genome-wide association studies, copy number variations, gene expression studies, and epigenetics while addressing the ethical, legal, and social issues to emerge from this new and exciting research approach.