In 1960, marriage was a virtual precondition for childbearing. By 1997, out-of-wedlock births accounted for 26 percent of fertility among whites and 69 percent among blacks. This paper presents a model that integrates economic theories of fertility and marriage to help understand the growth of out-of-wedlock childbearing. In the theory, fathers can shift the costs of child rearing to single mothers. If females are in excess supply and have sufficiently high incomes, a marriage market equilibrium may exist in which children are born within marriage to high-income parents, whereas in low-income groups men father children by multiple partners outside of marriage.