Studies on Europe and the U.S. indicate that marriage has been postponed, cohabitation has increased, and unions are more likely to dissolve. However, no study has been able to capture all of these dimensions simultaneously. Here we use latent class growth models to trace the complexity of union formation in the United States and 14 countries in Europe. We examine how union status can change between the ages of 15-45 for women born 1945-74. After determining the optimal number of latent classes, we calculate the probability of falling into each class by country and cohort. This shows the heterogeneity of union patterns across countries and over time. In all countries, changes in relationship patterns have been driven by the postponement of marriage, while premarital cohabitation and separation have varied more by region. Cohabitation has emerged as its own class but is not yet identical to marriage.