Prosociality is a multifaceted concept referring to the many ways in which individuals care about and benefit others. Human prosociality is foundational to social harmony, happiness, and peace; it is therefore essential to understand its underpinnings, development, and cultivation. This handbook provides a state-of-the-art, in-depth account of scientific, theoretical, and practical knowledge regarding prosociality and its development. Its thirty chapters, written by international researchers in the field, elucidate key issues, including: the development of prosociality across infancy, childhood, adolescence, and beyond; the biological, cognitive, emotional, and motivational mechanisms that underlie and influence prosociality; how different socialization agents and social contexts can affect children’s prosociality; and intervention approaches aimed at cultivating prosociality in children and adolescents. This knowledge can benefit researchers, students, practitioners, and policy makers seeking to nurture socially responsible, caring youth.