The study of adult development and aging is a rapidly growing field, as the older adult population continues to increase. This handbook provides a comprehensive overview of adult development and aging, including theory and methods, biological factors and health, personality and individual differences, cognition, mental health and clinical issues, variability and diversity in development, and social and contextual factors. Each chapter provides a state-of-the-art review of a different domain of adult development and aging and applies a lifespan developmental lens to the topic, addressing aspects of continuity and change in biopsychosocial processes across multiple aspects of adult development, with an emphasis on midlife and later adulthood. Distinguished scholars and researchers included in this handbook contribute a broad variety of perspectives, approaches, and methods, demonstrating the heterogeneity and diversity of adult development and aging. The volume provides foundational research and explication of theories as well as insight into current topics of growing importance within the field, such as alternative therapies including mindfulness and meditation; technology support for aging adults; the health of LGBTQ older adults; health inequities related to race, ethnicity, socioeconomic status, and other sociocultural factors; cultural variability in aging; and addressing ageism.