In language we sometimes find cases where grammatical rules fail to apply as expected (e.g. possessives in English are formed by adding -s, but if a word already has a plural -s the second one ‘disappears’.). These are often seen as idiosyncratic exceptions with no importance for linguistic theory. Such apparent deviations, however, are widely found across languages, constituting a challenging reversal of the usual model of grammar, which assumes that superficial aspects of language such as phonology or morphological form cannot influence its deeper structure. Unfortunately, these clashes are only imperfectly understood. Despite their widespread presence in language, for example, we still do not know why similar sequences can be banned in one language but allowed in another. The phenomenon is complex in many other ways. Morphological clashes need not just involve phonological identity or similarity, they may also be triggered by morphosyntactics feature alone, that is to say, identity of meaning or function. Furthermore, there is a wide range of elements (e.g. affixes, clitics or free words) which can be involved in these clashes, which may take place both under adjacency and at a distance, may involve derivational or inflectional morphology, may be “repaired” in different ways (e.g. by elision or disimilation) or not at all. These clashes also have important diachronic and cognitive ramifications, since a successful explanation of the phenomenon must involve a good understanding of its emergence and evolution, as well as of the cognitive and language processing mechanisms which make it possible. Despite its inherent interest and wide implications, the topic has not yet been explored in any depth. My proposal involves a novel approach, grounded in functionalist and typological explanations, covering every relevant aspect of the phenomenon. The goal will be to systematically gather original data from a representative sample of languages and to analyse and classify them according to the grammatical characteristics mentioned above. A better understanding of morphological clashes will add to the recent advances in other related phenomena (see Surrey Morphology Group's most recent contributions) and will contribute, consequently, to Morphological Theory as a whole.