This paper describes FamilyScape, an agent-based simulation model of family formation. The model’s phenomena of primary interest are the incidence of pregnancy and childbearing within and outside of marriage. We model the key antecedents of pregnancy – sexual activity, contraceptive use, and female fecundity – and many of its most important outcomes, including childbearing among married and unmarried parents, children’s chances of being born into poverty, and abortion. Realistic variation is simulated in the model’s behavioral inputs according to individuals’ demographic characteristics. We present diagnostic results demonstrating that, especially for unmarried individuals, the model produces rates of pregnancy and childbearing that match their real-world equivalents when the simulation’s inputs are carefully aligned to real-world data. The model readily lends itself to a wide range of policy applications, and we present illustrative results from a simulation that estimates the effects of an expansion in publicly-subsidized contraceptive services.