Dichogamy, the temporal separation of gender within a flower, occurs in two forms: protandry, male function before female function, and protogyny, the converse. Dichogamy is found in over 250 families and cannot be explained solely by its role in avoiding inbreeding. The primary goal of my thesis is to investigate the interference-avoidance hypothesis for the evolution of dichogamy. I accomplish this goal through a comprehensive, evolutionary evaluation of dichogamy, including functional, genetic, comparative, and theoretical analyses of this important floral character. To evaluate the function of protandry, I manipulated protandry and influorescence size in the plant 'Chamerion angustifolium' (Onagraceae) and monitored male and female reproductive success and pollen dispersal. Protandrous plants had a twofold siring advantage relative to adichogamous plants. However, this advantage did not increase linearly with inflorescence size. Furthermore, two negative consequences of simultaneous hermaphroditism were that anthers impeded pollinator's access to stigmas and pollinators spent more time foraging on hermaphroditic flowers, relative to female flowers. These functional analyses demonstrate that, in addition to reducing inbreeding, protandry can increase pollen export, thereby enhancing male fitness. I conducted genetic analyses of protandry in 'C. angustifolium ' using a paternal half-sib design and an artificial selection experiment. The duration of male phase was heritable ('h'2 = 0.27) and I detected no genetic correlation between male-phase duration and floral size. However, male-phase duration was positively correlated with floral display size. Moreover, male- and female-phase durations were negatively correlated. A conceptual model showed this gender trade-off produced fitness landscapes favourable to protandry's evolution. To gain a broader perspective on the evolution of dichogamy, I conducted a phylogenetic, comparative analysis of dichogamy and self-incompatibility. I found that protandry was positively correlated with self-incompatibility and protogyny with self-compatibility. Furthermore, dichogamy changes repeatedly throughout the phylogeny. These results support the interference-avoidance hypothesis for the evolution of protandry and inbreeding avoidance for protogyny. Collectively, this research suggests that protandry enhances male fitness through reductions in both within- and between-flower interference, while protogyny reduces inbreeding. This work has revealed unexpected fitness benefits to protandry and helps to explain the wide taxonomic and ecological distribution of this trait in flowering plants.