Hornbills (Aves: family Bucerotidae) are a charismatic group of Palaeotropical birds recognised for their distinctive morphology (casque) and behaviour (nest-sealing). Hornbill diet, habitat use, distribution and social system display pronounced interspecific variation, and their mutualistic interactions with tropical fruits provide vital ecosystem services. A wide range of species of hornbills across the Afrotropics, Indo-Malaya and Melanesia are of conservation concern. However, the evolutionary history of hornbills remains unclear and there are conflicting hypotheses about their origin, tempo of diversification and biogeography. Despite a comprehensive cladistic analysis of phenotypic data, there are unresolved taxonomic uncertainties within the family, and although a gradual accumulation of molecular data has revealed interesting phylogenetic relationships, methodological limitations, and incomplete sampling, has left gaps and produced incongruent results. Hence the evolutionary framework against which to interpret the diversity of this group is incomplete. The aim of this thesis was to construct a well-resolved molecular phylogeny of hornbills, and to use it to address longstanding questions about the evolution and diversification of these remarkable birds. This thesis presents a broad phylogenetic and biogeographic analysis of the family Bucerotidae, based on a coalescence of molecular methods and comparative analyses, largely using historical samples and recently-developed bioinformatic approaches. I provide the first complete species-level molecular phylogeny of hornbills, derived from nuclear adenylate kinase 1 intron 5 and mitochondrial (mtDNA) cytochrome b genes, and also a comprehensive mtDNA phylogeny covering 98% of the taxa, with extensive sampling of Asian geo-isolates. Using these two phylogenetic trees, I sought to determine the tempo of divergence, trace the evolution of traits, identify ancestral areas and colonisation routes, and also calculate genetic divergence. In this part of the work, I stress the importance of (1) using historical samples, (2) calibrating time-trees with fossil and molecular anchor-points, and (3) the use of a complete tree to test models of diversification and reconstruct ancestral states. My findings confirm the monophyly of Bucerotidae, showing nearly distinct African and Asian lineages, with relatively congruent topologies across different phylogenetic methods and genes. In turn, these gene trees were comparable with previous cladistic analysis based on phenotypic data. As a result, I was able to resolve some taxonomic issues and propose generic changes. Comparative analyses of social behaviour revealed that cooperative breeding is an ancestral trait, and its evolution in hornbills (in contrast with some other taxa) is associated with stable environments in combination with frugivory, territoriality and reduced body size. Analysis of the evolution of diet with diversification showed that the shift to frugivory from faunivorous African ancestors influenced the rapid diversification of Afro-Asian forest hornbills, facilitated by their preference for humid forests and mutualistic interactions with , hornbill-dispersed fruits (HDF). This dispersal of frugivorous lineages via the Palaeotropical Biotic Interchange promoted colonisation of Asia, with Sundaland being the centre of radiation for continental and insular Asian species. The gradual eastward colonisation from India to Melanesia matches palaeo-tectonic events that allowed dispersal across Huxley's, Wallace's and Lydekker's lines, and was congruent with historical biogeography of some HDFs. Finally, I used a combined analysis of genetic divergence and a standard scoring system for phenotypic data of Asian geo-isolates to provide evidence for quantitative delineation of species and propose changes in conservation status. My findings reveal the evolutionary history of horn bills from their emergence in the Late Oligocene from African origins, with a switch to frugivory influencing successful colonisation of hornbills and HDFs in Asian forests, which combined to promote sociality. Overall, this thesis demonstrates that access to novel environments and innovations to ecological niche facilitate rapid diversification in an avian lineage, and that this process is further promoted by the interplay of these birds in complex mutualistic interactions with their food, as well as palaeo-climatic and palaeo-tectonic changes.