Social psychology is commonly defined as the study of the social aspects of cognition, emotion, and behavior. This chapter traces the history of this field of study in the United States from the late nineteenth century to the present. Early twentieth-century social psychologists sought a field of inquiry that answered the pressing social questions of the day, suited the individualistic climate of the United States, and corresponded with the ethos of the new scientific psychology. By the 1920s, many psychologists were practicing a social psychology that focused on the scientific study of the social aspects of individual life. In the years between the two World Wars, the field expanded, with the founding of the Society for the Psychological Study of Social Issues, the application of social psychological research to real-world problems, and the expansion of research topics. World War II brought funding, interdisciplinary collaboration, and expanded research opportunities, resulting in unprecedented growth for social psychology. By the 1950s, the field was well established as a permanent and prominent part of the landscape of the behavioral sciences. Though its subject matter, practices, and methods have repeatedly been the subject of debate and critique, social psychology continues to thrive as a subfield of psychology.