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This chapter provides an introduction to and brief overview of empirical and theoretical work on sexual confl ict in humans, some of which is showcased in the current volume. We begin the chapter with a brief review of evolution by natural selection. We then discuss the application of evolutionary principles to psychology, with a focus on human psychology. With this background established, we present an overview of theory and research on sexual confl ict in humans. Sexual confl ict was a recurrent feature of human evolutionary history, just as it has been in every sexually reproducing species that does not practice lifelong genetic monogamy. One source of much of the confl ict between men and women can be reduced to an asymmetry in reproductive biology: Fertilization and gestation occur within women, not men. This asymmetry produces (1) sex diff erences in minimum obligatory parental investment and (2) paternity uncertainty, but maternity certainty. These consequences of internal fertilization and gestation help to account for many phenomena in humans, including sexual coercion, commitment skepticism, sexual overperception, and a host of adaptations associated with sperm competition. Key Words: sexual confl ict in humans, infi delity, paternity uncertainty, sperm competition An understanding of sexual confl ict, in general, and human psychological and physiological adaptation to sexual confl ict, in particular, requires familiarity with the basics of evolutionary theory. Many excel-lent introductions to evolution exist to which w