Human children are unique among primates for the amount and duration of care and provisioning they require from adults during infancy. Studies in anthropology, demography and human behavioural ecology show considerable flexibility across human societies in who provides this care, and a wide variation in how it impacts child wellbeing. However, there is still relatively little research that determines which factors predict who provides this help. Studies exploring allomaternal support in detail tend to have small sample sizes, whereas large-scale studies have used proxy measures for allomaternal care (e.g., absence/presence of kin). Such dichotomous indicators may mask variation in children’s caregiving environments, and thus in the relationship between children’s care and health. This thesis has three main aims: (1) to draw on the methodological strengths of anthropological and demographic research on childcare to collect data that overcomes previous limitations; (2) to contribute to a more thorough and interdisciplinary understanding of the patterns and predictors of childcare; and (3) to shed light on potential demographic and health implications of childcare. These aims are addressed in three research chapters, using detailed data collected on parental and alloparental childcare from 808 children aged under 5-years, in two diverse communities in north-western Tanzania representative of on-going demographic and urban transitions. This research confirms that children receive many different types of care from a variety of individuals, including parents, siblings, various relatives and non-kin. Who cares for children is determined by factors including the child’s sex, parental co-residence, and the community’s level of market integration; with suggestions of responsive childcare systems. However, relationships between allomaternal care and children’s health are inconsistent, implying complexity in children’s caregiving environments. This research also highlights the value of understanding how allomaternal behaviour changes with market integration, shedding light on contemporary demographic and health transitions.