AbstractResearch literature in the field of extramarital sex (EMS) is reviewed. Initially, definitional problems which reveal the need for increased rigor in specifying the sexual behavior under consideration, the dyad or group outside of which the behavior occurs, and the consensual or non‐consensual nature of the behavior are discussed. Twelve surveys of EMS are examined and the limitations of incidence rate figures discussed. Empirical studies have attempted to identify key variables which discriminate between EMS and non‐EMS samples. The findings of this research are summarized according to four variable categories: social background characteristics, characteristics of the marriage, personal readiness characteristics, and sex and gender differences. Characteristics of the marriage and personal readiness characteristics are found to be of prime importance in understanding EMS, although sex and gender differences frequently qualify major empirical relationships. The attitude continuum of extramarital sexual permissiveness (EMSP) has received considerable research attention. Although premarital sexual permissiveness is the most significant correlate of EMSP, the tenuous results of research examining the attitude‐behavior (EMSP‐EMS) link demonstrate the need for new conceptualizations in this area. Finally, the conclusions of this review are presented in summary.