Family structure refers to the organization of individuals who are related by blood, marriage, adoption, or strong social bonds into a household. In the United States, family structure has been through two dramatic shifts in the past 160 years. The first shift occurred between 1850 and 1950 and saw the decline of the vertically extended household that included grandparents, parents, and children in the same dwelling and the rise of the nuclear family household, which included married biological parents and full siblings only. Since then, family structure has transitioned into a plurality of forms. Factors that contribute to family structure change since 1950 include delayed marriage, divorce, remarriage, cohabitation, nonmarital fertility, family formation among gay and lesbian couples, and population aging. These changes in family structure require resilience in families experiencing them and increased complexities for interacting with the health care system. Health professionals also need to adapt to these new structures as they partner with families to promote family health.