Extradyadic involvement (EDI) refers to physical or emotional intimacy that takes place outside an existing romantic relationship. When EDI violates relationship standards of exclusivity, infidelity is said to have occurred. Although EDI and infidelity are both fairly prevalent, their underlying causes have not been well understood historically, and many questions remain about why some individuals become extradyadically involved while others do not. Prior research has uncovered several factors that might contribute to EDI, perhaps the most notable of which have been individual attitudes, relationship quality, and contextual opportunity. General trends suggest that forbidding beliefs, high relationship quality, and low contextual opportunity serve to diminish the likelihood of EDI. However, effects have by no means been unequivocal, in large measure because of the methodological limitations of earlier work. Specifically, the predictors of interest have not been appropriately examined within a longitudinal framework, nor have they typically been integrated into a unified theoretical model. Moreover, opportunity and beliefs have almost uniformly been assessed using measures with questionable psychometric properties, whereas relationship quality has been operationalized differently from one study to the next. In an effort to redress these limitations, and thereby better elucidate the underlying determinants of EDI, the current study integrated relationship quality, individual beliefs, and contextual opportunity into a unified theoretical model of EDI development and tested its validity within a longitudinal framework. Relationship quality was operationalized in terms of commitment, as defined by the Investment Model, and beliefs and opportunity were evaluated using newly devised scales exhibiting sound measurement properties. As predicted, lower opportunity, along with more forbidding beliefs about extradyadic participation, diminished the likelihood of subsequent EDI. Importantly, the effects remained even after controlling for the level of extradyadic engagement observed at time 1. In contrast, commitment had no discernable impact on the development of EDI. However, because of a restricted range in commitment, its non-significant effect may have been spurious.