Prosocial behavior first appears in the second year of life. How can prosociality so early in life be explained? One possibility is that infants possess specialized cognitive and/or social capacities that drive its emergence. A second possibility is that prosocial behavior emerges out of infants' shared activities and relationships with others. These possibilities have motivated a number of current explanatory efforts, with a focus on two complementary questions. First, what is evolutionarily prepared in the very young child and how does it give rise to prosocial behavior? Second, how do proximal mechanisms, including social experiences, contribute to the early development of prosociality? The papers in this special issue represent some of the most recent work on these questions. They highlight a diverse array of new methods and bring them to bear on the nature and development of early prosocial understanding and behavior.