Infidelity is a major cause of divorce and spousal battering. Little is known, however, about which individuals are susceptible to infidelity, or about the relationship contexts that promote infidelity. This study of 107 married couples examines three sets of possible predictors of infidelity: Personality factors such as narcissism and conscientiousness; relationship contexts, including recurrent sources of conflict and sexual satisfaction; and the relative “”mate value”” of the individuals composing a couple. We obtained self-report and spouse-report data on susceptibility to infidelity. We obtained self-report, spouse-report, and interviewer-report data on personality, relationship context, and relative mate value. Personality factors most strongly linked to susceptibility to infidelity were low Conscientiousness, high Narcissism, and high Psychoticism. Relationship contexts most strongly linked to susceptibility to infidelity include sexual dissatisfaction, and specific sources of conflict such as partner complaints about jealousy. Discussion addresses limitations of this study and directions for future research on predicting infidelity.