Research conducted in the last fifteen years has placed in question many of the traditional conclusions scholars have formed about human female sexuality. Though conventional wisdom asserts that women's estrus has been evolutionarily lost, Randy Thornhill and Steven W. Gangestad assert that it is present, though concealed. Women, they propose, therefore exhibit two sexualities each ovulatory cycle-estrus and sexuality outside of the estrous phase, extended sexuality-that possess distinct functions. Synthesizing research in behavioral evolution and comparative biology, the authors provide a new theoretical framework for understanding the evolution of human female sexuality, one that is rooted in female sexuality and phylogeny across all vertebrate animals.