Throughout human history and around the world, co-sleeping was the context for human evolutionary development. Currently, most of the world’s peoples continue to practice co-sleeping with infants, but there is increasing pressure on families in the West not to co-sleep. Research from anthropology, family studies, medicine, pediatrics, psychology, and public health is reviewed through the lens of a developmental theory to place co-sleeping within a developmental, theoretical context for understanding it. Viewing co-sleeping as a family choice and a normative, human developmental context changes how experts may provide advice and support to families choosing co-sleeping, especially in families making the transition to parenthood. During this transition, many decisions are made by parents “intuitively” (Ball, Hooker, & Kelly, 1999), making understanding the developmental consequences of some of those choices even more important. In Western culture, families are making “intuitive” decisions that research has shown to be beneficial, but families are not receiving complete messages about benefits and risks of co-sleeping. Co-sleeping can be an important choice for families as they make the life-changing transition to parenthood, if individualized messages about safe infant sleep practices (directed toward their individual family circumstances) are shared with them.