[Truncated abstract] The assumption that facial attractiveness signals mate quality is central to current evolutionary theories of human sexual selection. Evidence for direct links between attractiveness and mate quality is, however, scarce, and the exact nature of mate quality remains the subject of debate. Mate quality may include genetic diversity, because genome-wide diversity has been linked to individual fitness, and diversity within the Major Histocompatibility Complex (MHC) has been associated with immunocompetence and health in many species. This thesis investigates whether individual genetic diversity plays a role in human sexual selection. The main aim is to examine whether MHC diversity, compared to genetic diversity in general, is especially important for mate preferences, health and mating success. The four studies herein are based on data collected from a large sample of heterosexual, Caucasian males and females. Participants were photographed, provided a DNA sample, and completed questionnaires regarding sexual history and health. Genetic diversity was calculated as both mean heterozygosity (H) and standardised mean-d2 (d2), separately for 12 MHC microsatellite loci and 11 nonMHC loci. The photographs were rated for various attractive features by opposite-sex raters. The first study investigated whether MHC diversity influences preferences for facial appearance in a potential mate, and if so, are they specific to the MHC and are they mediated by specific facial characteristics? I found that MHC-H, but not nonMHCH, positively predicted male facial attractiveness, and that this relationship was mediated by facial averageness. For females, nonMHC-d2 predicted facial symmetry, and potentially attractiveness. These findings indicate that faces contain visual cues to mate quality in both males and females, providing support for evolutionary theories that our preferences are adaptations for identifying mates of high quality. ... Measuring them both allowed me to tease apart their effects on mate preferences, and on health and mating success. Indeed, the MHC appears to be especially important in sexual selection as MHC diversity predicted female mate preferences after controlling for nonMHC diversity, and MHC dissimilarity predicted male mate preferences after controlling for nonMHC dissimilarity. Moreover, although MHC diversity did not appear to influence males’ preference for females, it did predict female mating success, suggesting that males also attend to MHC-related cues, although perhaps non-facial cues, when seeking mates. Additionally, nonMHC diversity predicted both male preferences for female faces and health, suggesting that such preferences are adaptive. Importantly, by providing direct links between facial attractiveness and biological markers of individual quality, genetic diversity, these results support the commonly held assumption that facial attractiveness signals mate quality.