Our world is changing rapidly and factors like urbanisation, changed agricultural practices and climate change are causing losses in butterfly diversity. It is therefore of importance to understand the source of their diversity. Given the remarkable diversity of herbivorous insects compared to their non-herbivorous sister groups, changes in host use have been implicated as a promoter of speciation. This thesis looks at geographical aspects of host range evolution and the plasticity of host use. We show that butterflies in the subfamily Nymphalinae that feed on a wide range of host plants have larger geographic ranges than species with narrower host ranges. Although tropical butterflies appear to be more specialised than temperate species, this effect is lost when controlling for the differences in geographic range. Geographic variation in host plant use within Polygonia faunus, related to morphologically distinct subspecies, did not show any genetic differentiation. This suggests that the observed variation in host plant use is a plastic response to environmental differences. Reconstructing host use for the Polygonia-Nymphalis and Vanessa group shows that plasticity is also important for understanding host use at the level of butterfly genera. Using unequal transition costs and including larval feeding ability revealed that frequent colonisations of the same plant genus can often be explained by non-independent processes, such as multiple partial losses of host use, recolonisation of ancestral hosts, and parallel colonisations following a preadaptation for host use. These processes are further reflected in the conservative use of host plant orders within the butterfly family Nymphalidae. Few taxa feed on more than one host plant order, and these expansions occur at the very tips of the tree, which we argue is evidence of the transient nature of generalist host use. These insights improve our understanding of how host range evolution may promote diversification. / At the time of the doctoral defence,the following papers were unpublished and had a status as follows: Paper 3: Submitted; Papers 4 and 5: Manuscripts