Infidelity is often defined as perceived as a violation of relationship exclusivity rules, and many view infidelities as immoral. Thus, one can imagine the importance of understanding why up to a fifth of research samples report cheating in their current relationships. There is considerable literature regarding what factors predict infidelity. Although individual characteristics associated with infidelity do exist, relationship-based variables tend to be the most consistent and robust factors. This chapter will discuss various theories and research findings that suggested different relationship-based factors and frameworks with which to consider and predict why some people engage in infidelity. This chapter reviews some major ideas and research through the lens of the investment model of commitment (i.e., satisfaction, investment, quality of alternatives, commitment), as well as other relationship-based variables (e.g., opportunities and boredom, relationship type and length) that have received empirical support. Finally, the chapter ends with a nod to future directions in this area, and a notion of what researchers should expect from the literature in the future.