Theories of social gerontology have progressed from a focus on individuals’ later-life decline to theories that emphasize the intra- and interindividual variability of later-life experiences and the ways in which such heterogeneity is conditioned by social structural, cultural, and interpersonal factors that often begin in childhood and continue to shape individuals and members of their social networks across the life course. Consistent with theories across the sciences, theories of social gerontology predict and explain real-world experiences. In the case of social gerontology, the goals of theory address a wide array of phenomena, ranging from individuals’ attitudes and motivations, social networks and social support, the actions and functions of formal organizations, the embodiment of cultural norms and stereotypes, social determinants of health, and sources of inequality throughout the life course.. As the field of social gerontology has developed, theories in the field have shown increasing complexity, particularly regarding the roles of early life course experiences, social structural positions, and interpersonal relations in explaining variations in well-being, longevity, and the quality of life across the lifespan. As part of this increased complexity, social gerontology has become increasingly cross-disciplinary, spanning disciplines such as sociology, psychology, biology, anthropology, public health, medicine, and engineering, with a strong emphasis on how each discipline can contribute to developing principles that transcend individual fields. These integrative theories of social gerontology are crucial to developing comprehensive approaches to improving the health and well-being of individuals throughout the life course. Theories of social gerontology help us comprehensively understand the aging process by emphasizing individual characteristics, social relationships, and the larger cultural contexts in which individuals’ lives are embedded.