Although much has been learned from cross-sectional research on marriage, an understanding of how marriages develop, succeed, and fail is best achieved with longitudinal data. In view of growing interest in longitudinal research on marriage, the authors reviewed and evaluated the literature on how the quality and stability of marriages change over time. First, prevailing theoretical perspectives are examined for their ability to explain change in marital quality and stability. Second, the methods and findings of 115 longitudinal studies--representing over 45,000 marriages--are summarized and evaluated, yielding specific suggestions for improving this research, Finally, a model is outlined that integrates the strengths of previous theories of marriage, accounts for established findings, and indicates new directions for research on how marriages change.