This chapter reviews the biobehavioral interactions of breastfeeding. It then uses historical and cross-cultural data to demonstrate how these bio-behavioral interactions have been shaped by social and cultural factors into the wide variety of infant feeding practices extant today. Breastfeeding is better viewed as a behavioral domain than as a single feeding behavior. Breastfeeding is similar to other forms of eating in that the variation in its component behaviors is regulated by the social and cultural milieu in which the participants interact. Yet breastfeeding is distinct from other domains of feeding behaviors in one critical aspect. Variations in its component behaviors have clear biological ramifications for the entire lactation process, affecting everything from the amount of milk available to the child, to the nutrient content of the milk, to the nutritional and reproductive status of the mother. Breastfeeding of any kind depends on early initiation of suckling.