This article argues that marriage has undergone a process of deinstitutionalization—a weakening of the social norms that define partners' behavior—over the past few decades. Examples are presented involving the increasing number and complexity of cohabiting unions and the emergence of same-sex marriage. Two transitions in the meaning of marriage that occurred in the United States during the 20th century have created the social context for deinstitutionalization. The first transition, noted by Ernest Burgess, was from the institutional marriage to the companionate marriage. The second transition was to the individualized marriage in which the emphasis on personal choice and self-development expanded. Although the practical importance of marriage has declined, its symbolic significance has remained high and may even have increased. It has become a marker of prestige and personal achievement. Examples of its symbolic significance are presented. The implications for the current state of marriage and its future direction are discussed.