Data from the 1987–1988 National Survey of Families and Households are used to provide national estimates of cohabitation trends and levels. The rapid increase since around 1970 is documented over both birth cohorts and marriage cohorts. Almost half of the persons in their early 30s and half of the recently married have cohabited. Changes in the proportion ever married are compared with changes in the proportion who have either married or cohabited. Much of the decline in marriage has been offset by increased living together without being married. The stability of unions of various types is compared. Cohabitations end very quickly in either marriage or disruption. About 60 percent of all first cohabitations result in marriage. Cohabiting unions and marriages preceded by cohabitation are much more likely to break up than are unions initiated by marriage. Multivariate analysis reveals higher rates of cohabitation among women, whites, persons who did not complete high school, and those from families who received welfare or who lived in a single-parent family while growing up.