Researchers largely have relied on a measure of family structure to describe children's living arrangements, but this approach captures only the child's relationship to the parent(s), ignoring the presence and composition of siblings. We develop a measure of family complexity that merges family structure and sibling composition to distinguish between simple two-biological-parent families, families with complex-sibling (half or stepsiblings) arrangements, and complex-parent (stepparent, single-parent) families. Using the Survey of Income and Program Participation (SIPP), we provide a descriptive profile of changes in children's living arrangements over a 13-year span (1996-2009). SIPP sample sizes are sufficiently large to permit an evaluation of changes in the distribution of children in various (married, cohabiting, and single-parent) simple and complex families according to race/ethnicity and parental education. The article concludes by showing that we have reached a plateau in family complexity and that complexity is concentrated among the most disadvantaged families.