This chapter studies the behavior of one specific group of extended family members, namely aunts and uncles. Aunts and uncles typically belong to the same generation as the children’s parents, but obviously the relations between aunts and uncles and their nieces and nephews are very different compared to parent–child relations. This is due to the fact that aunts and uncles are seldom the main caregivers of their nieces and nephews, and in contemporary Western societies, they rarely live in the same household as them. That said, however, noncoresiding aunts and uncles can serve as important alloparents for children and are often highly committed to the lives of their nieces and nephews, providing, for instance, social support, friendship, mentoring, and role models. The chapter then outlines key evolutionary theories of intergenerational family relations explaining the behavior of aunts and uncles. It also considers the investment of aunts and uncles in contemporary affluent societies