New in paperback. Hardcover was published in 2023. 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.