During the last 40 years, family organization in the US has become increasingly complex. In 1970, 85 percent of children lived with both married parents; by 2009, 56 percent resided with both married parents and no step- or half-siblings. The balance, comprising 44 percent of today’s children, experience a multiplicity of non-traditional family arrangements that may include having at least one parent’s residence outside the child’s household, parents’ changes in union status with new partners, or the inclusion of step- or half-siblings. This family complexity produces an intricate and evolving network of biologically and socially-based family relationships that transcend household boundaries. Family complexity is associated with children receiving fewer economic resources from parents and other kin, with consequences for their well-being across the early life course. However, existing research has not accounted for the multiple dimensions of family complexity simultaneously; nor has it fully accounted for the dynamic nature of family complexity or for the antecedents to family complexity that might explain its association with resource distribution to children. As a result, research findings about this association are fragmented and potentially misspecified. We address these limitations by applying rigorous statistical methods to data from a multigenerational national study to focus on a critical phase of the life course: the transition to adulthood. In particular, we ask whether and how exposure to family complexity in childhood shapes the family resources available to children as they embark on a sequence of consequential and intensive life events including homeleaving, labor force entry, educational attainment, and family formation. We focus on three components of family complexity: union instability, multipartner fertility, and nonresidential status. We ask how these dimensions of family complexity operate independently and in concert to shape the distribution of available family-based resources over children’s early life course through the transition to adulthood. We use data from the 1968-2015 Panel Study of Income Dynamics and its supplements to connect children to mothers’ and fathers’ union and childbearing trajectories from prior to the child’s birth through the transition to adulthood even where children have little or no contact with nonresident parents. This design allows us to 1.) provide new descriptive estimates of family relationships and resources that span households, 2) assess the factors that play into resource allocation in complex families, and 3) connect time-varying measures of resource allocation in complex families to the transition to adulthood net of a rich set of background information and controls. Our analytic models improve estimates of how the unfolding and dynamic relationship between family complexity and family economic resources during childhood impacts children’s transition to adulthood by reducing bias in estimators through adjustments for time-dependent confounding between family complexity and family economic resources. The family complexity database we construct from this project will be available online as a public release file at the PSID Data Center.