Darwin's theory of sexual selection explains the existence of sexual dimorphism, or within-species sex differences in shape, color, size, and behavior. In some cases, sexually dimorphic traits, especially extravagant male ornaments, seem maladaptive and thus in opposition to natural selection. The crux of Darwin's theory was that sexual selection arises from individual differences in reproductive success that result from competition for mates. In this dissertation, I investigated several aspects of sexual selection and the evolution of female mating preferences and male ornaments in the great frigatebird (Fregata minor). Frigatebirds as a group (family Fregatidae) are the most ornamented of any seabirds, and are among the most ornamented of any animal group. Their most prominent ornament is a gular (throat) pouch which becomes red in males during the breeding season, and which is inflated and displayed to females during courtship. Male courtship display also includes a warble vocalization and extension and trembling of the wings. I investigated the following issues concerning sexual selection and ornamentation in great frigatebirds: 1) the source of ornamental coloration in male great frigatebird gular pouches. I determined that this was a carotenoid-based color display; 2) the relationship of male mating success to gular pouch size and coloration. I determined that mating success was not related to the size or color of this ornament; 3) the relationship between male vocal display traits and female preferences. Again, I found no relationship between vocal display traits and female preferences, and finally, 4) the role of a major histocompatibility complex (MHC) locus in female mate choice. The MHC is a highly polymorphic multi-gene family associated with immune defense and has been proposed to play a role in mate choice. I found a significant disassortative mating pattern amongst mated pairs compared to random pairings based on MHC genotypes. In summary, I found no evidence for female mating preferences based on visual or auditory display traits associated with male ornamentation. However, I did find evidence for female mating preferences based on genetic dissimilarity at an MHC locus.