The prevalence of nonmarital cohabitation is steadily increasing in the United States. In evaluating the contribution of this new living arrangement to family formation, analysts have relied primarily on comparisons between individuals who cohabit and those who do not. We complement this line of inquiry by comparing the United States and 16 industrialized nations. We first identify six conceptually distinct ideal types of cohabitation with respect to family formation. We then propose empirical indicators to distinguish between the different ideal types, and estimate the values of these indicators for each of the 17 nations. Our findings indicate that although a number of countries fit an empirical pattern corresponding to one ideal type, cohabitation in the United States is more difficult to characterize.