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This thesis investigates the origins and behaviour of the non-canonical morphological phenomenon of heteroclisis in the verb paradigms of Latin and the Romance languages. Heteroclisis is the coexistence, within a single paradigm, of forms which pattern according to different inflectional classes existing otherwise in the language: a heteroclite lexeme can thus be seen as 'mixed' or 'undecided' as to its inflectional identity. I begin by examining the development of the theoretical concept of heteroclisis and approaches to the idea of inflectional class in general, before situating heteroclisis in typological space in comparison with better-known instances of non-canonical morphology such as deponency and suppletion; heteroclisis exists at a different level of generalization from these, because its identification presupposes the existence of inflectional classes, themselves generalizations over the behaviour of individual lexemes. I also consider two recent theoretical treatments of the phenomenon and survey recent linguistic studies making use of the notion. I then look at the synchronic and diachronic behaviour of heteroclisis in Latin and Romance verbs: the great time depth of our attestations of these languages gives us the chance to witness the development of successive examples of heteroclisis, and their subsequent treatment within the morphological system, in the history of a single family. Focusing chiefly on data from Latin, Romanian and Romansh, I find that the principal (though not the only) source for new instances of heteroclisis in Latin/Romance lies in regular sound change, and find that speakers can treat these synchronically anomalous patterns as robust models of inflectional behaviour to be extended over the lexicon or brought into line with pre-existing types of paradigm-internal alternation. These findings concur with previous demonstrations that speakers make use of non-canonical phenomena as markers of the internal structure of inflectional paradigms.