Species are the fundamental unit of biodiversity, and the evolution of new species is a fruitful area of research. In sexually reproducing organisms, reproductive isolation is considered fundamental to speciation, and the evolution of mate choice is thus an important mechanism of species origin and maintenance. In this dissertation, I focus on the butterfly genus Heliconius to elucidate how mate choice has contributed to speciation. I first simulated genomic divergence in different demographic and selective scenarios to understand whether a nonlinear increase in the highly differentiated part of the genome over time denotes the actions of selection and gene flow during speciation. This pattern, first noted among species of Heliconius, arises in a wide range of scenarios, including neutral divergence with no gene flow, while high gene flow and selection disrupt it. Next I tested mate preference of live butterflies of two species, H. cydno and H. pachinus, that regularly exchange genes. I found a preference for conspecific mates in both sexes, and thus that both sexes contribute to premating reproductive isolation. This is one of the first demonstrations of interspecific female choice in Heliconius. I further examined hybrid males' preference for both parental species and found that, contrary to theoretical predictions, preference was not correlated with two wing colour pattern elements thought to play a role in mate choice. Finally, I conducted a meta-analysis of premating isolation among Heliconius species and subspecies, finding small effects of both genetic differentiation and experimental design. The long history of research on genomics and behaviour of Heliconius butterflies continues to shed light on the role of mate choice in speciation and reveal directions for future research.