The temptation to engage in extradyadic sex and the tendency to respond with jealousy to such behavior by one's partner are interrelated and universal human phenomena, which are part of our human evolutionary heritage. In the present chapter, we first discuss the norms with respect to extradyadic sex, the incidence of extradyadic sexual involvement, and determinants of such involvement, for example attitudes towards sexuality, attachment styles, and relationship factors. In the second part of the chapter we focus on the consequences of extradyadic involvement for the individual and the primary relationship, with a particular focus on the experience of jealousy. After discussing several types of jealousy, we discuss the determinants of jealousy, such as the type of infidelity, the characteristics of the romantic rival, and the influence of hormones. The final part of the chapter is devoted to coping with extradyadic sex and jealousy as well as therapeutic approaches.