Emerging data suggest that during childhood, close family relationships can ameliorate the impact that adversity has on life span physical health. To explain this phenomenon, a developmental stress buffering model is proposed in which characteristics of family relationships including support, conflict, obligation, and parenting behaviors evolve and change from childhood to adolescence. Together, these characteristics govern whether childhood family relationships are on balance positive enough to fill a moderating role in which they mitigate the effects that childhood adversities have on physical health. The benefits of some family relationship characteristics are hypothesized to stay the same across childhood and adolescence (e.g., the importance of comfort and warmth from family relationships) whereas the benefits of other characteristics are hypothesized to change from childhood to adolescence (e.g., from a need for physical proximity to parents in early childhood to a need for parental availability in adolescence). In turn, close, positive family relationships in childhood operate via a variety of pathways, such as by reducing the impact that childhood stressors have on biological processes (e.g., inflammation) and on health behaviors that in turn can shape physical health over a lifetime.