Infidelity toward obtaining a new partner is commonly abetted by the tendency, especially during the peak reproductive years, to continually assess other potential companions’ mate value, suitability, and availability. The strategy of maintaining a relationship while seeking other partners can be adaptive for several reasons: (1) Whether in service of finding a new partner or not, infidelity offers the male more potential childbearing partners and provides the female a means to obtain genes from a partner with a superior phenotype, (2) extradyadic sex potentially creates a bond with a new partner and can add to one’s perceived security in a relationship while paradoxically easing leaving it, and (3) infidelity can act to mitigate the financial or emotional risks of leaving a partner who has become less desirable. Mate switching infidelity is abetted by many intrinsic factors such as males’ tendency to reflexively attend to those with salient sexual features and females’ propensity to remain vigilant for new partners who offer superior genes, status, and resources. This chapter details these and several other cultural and evolutionary forces that underlie the mate switching hypothesis of infidelity. It also presents case studies to illuminate the psychological and clinical implications of this practice.