The initially puzzling phenomenon of substitute parenting in Homo sapiens falls into three broad categories that require distinct treatments. One major subtype of substitute parenting entails genetic relatives, especially grandparents, stepping up to replace parents who cannot or will not care for their children, and promoting their own inclusive fitness by so doing. A second subtype is stepparenthood, which is most persuasively interpreted as a component of "mating effort". Both stepparenting and replacement care by genetic relatives are cross-culturally ubiquitous and almost certainly ancient, and the behavior of substitute parents in these contexts is therefore likely to exhibit evolutionary adaptation to the characteristic opportunities and pitfalls associated with these recurrent social dilemmas. The same cannot be said, however, for the third major subtype of substitute parenting, namely adoption by non-relatives. Families sometimes adopt children to fill otherwise vacant social and familial roles or niches, and they foster or adopt children as a component of reciprocity and citizenship within close-knit communities. It is the modern practice of "adoption by stranger" that presents the greatest challenge to a simple conception of human beings as evolved fitness maximizers, by necessitating that we ask why large numbers of people elect to treat unrelated children as if they were their own. Each of these three broad categories of substitute parenting and their possible explanations will be discussed in a subsequent section of this chapter.