Cohabitation and marriage in the United States are converging relationships for those cohabiters who eventually marry. Using the “National Survey of Families and Households” and the “National Survey of Family Growth” as data sources, this dissertation examines trends over time in cohabitation and the types of people who cohabit before marriage, differences in behavior across relationship stages, and the impact of age at entrance into cohabitation on later divorce probability. Between 1965 and 2002 premarital cohabitation has become a more common and longer lasting relationship stage, and those who do not cohabit with their partner before marriage are an increasingly select group. Prior research has compared all cohabiters to all married couples and theorized that entrance into marriage is accompanied by a significant shift in behavior. Distinguishing between cohabiters with uncertain and definite marriage plans, recently married couples and those in longer term marriages, and excluding those who did not cohabit before marriage from comparisons is a more accurate way of determining if entrance into marriage affects the behavior of premarital cohabiters. Utilizing these comparison groups yields findings that entrance into marriage among premarital cohabiters is not accompanied by as significant of a change in behavior as has been found by prior research, and marital longevity in some cases affects behavior more so than entrance into marriage. The specific areas examined include work, wealth, debt, health and healthy behavior, and the gendered division of labor, including an examination of both paid and unpaid work. Finally ‘counting’ the start of the marriage at cohabitation for premarital cohabiters and taking into account the young age at which premarital cohabiters select and form unions with their partners explains a large portion of the effect of premarital cohabitation on divorce. Premarital cohabitation is then best described as a ‘probationary marriage’ and premarital cohabitation and marriage should not be conceptualized as distinct types of relationships, but as distinct stages of the same relationship.