In this chapter we review literature on both sexual and emotional jealousy. Jealousy is conceptualized as a cognitive, emotional, and behavioral response to a relationship threat. In the case of sexual jealousy, this threat emanates from knowing or suspecting that one’s partner has had (or desires to have) sexual activity with a third party. In the case of emotional jealousy, an individual feels threatened by her or his partner’s emotional involvement with and/or love for a third party. The experience and expression of jealousy is influenced by a number of factors, including culture, personality, and relational characteristics. Researchers taking an evolutionary perspective have also investigated and supported three hypotheses related to sex differences. First, men are more upset in response to sexual jealousy, whereaswomen aremore upset in response to emotional jealousy. Second, men become especially jealous when a rival is high in status-related attributes such as dominance and wealth, whereas women become especially jealous when rivals are physically attractive. Third, men are more likely than women to express jealousy by engaging in behaviors such as displaying resources and competing physically with the rival, whereas women are more likely than men to enhance physical attractiveness. The evolutionary perspective on sex differences in jealousy has been challenged by those who support cognitive-interpretative and/or social structural frameworks for explaining sex differences in jealousy. Nonetheless, social evolutionary theory appears to provide a good theoretical starting point for studying jealousy. Synthesizing this work with research and theory related to individual, cultural, and relational factors would provide an even richer understanding of both sexual and emotional jealousy.