In this article we review the advances made in the 20th century in studying marriages. Progress moved from a self-report, personality-based approach to the study of interaction in the 1950s, following the advent of general systems theory. This shift led, beginning in the 1970s, to the rapid development of marital research using a multimethod approach. The development of more sophisticated observational measures in the 1970s followed theorizing about family process that was begun in the decade of the 1950s. New techniques for observation, particularly the study of affect and the merging of synchronized data streams using observational and self-report perceptual data, and the use of sequential and time-series analyses produced new understandings of process and power. Research in the decades of the 1980s and 1990s witnessed the realization of many secular changes in the American family, including the changing role of women, social science's discovery of violence and incest in the family, the beginning of the study of cultural variation in marriages, the expansion of the measurement of marital outcomes to include longevity, health, and physiology (including the immune system), and the study of co-morbidities that accompany marital distress. A research agenda for the 21st century is then described.