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This thesis investigates the forms, functions and behaviour of tone in the phonology,lexicon, morphosyntax and the phonology-grammar interfaces in Ikaan (Benue-Congo, Nigeria). The analysis is based on an annotated audio corpus of recordings from 29 speakers collected during ten months of fieldwork complemented with participant observation and informally collected data. The study demonstrates that tone operates at a wide range of levels of linguistic analysis in Ikaan. As phonemes, tones distinguish meaning in minimal pairs and are subject to phonological rules. As morphemes, tones and tonal melodies bear meaning in inflection, derivation and reduplication. In the syntax, tones mark phrase boundaries. At the phonology-semantics interface, constructionspecific constraints on the tonal representation distinguish between predicating and referential nominal modifiers. Combined with intonation and voicing, tones distinguish between statements and morphosyntactically identical yes/no questions. The research identifies a range of unusual tonal behaviours in Ikaan. The two tones H and L follow markedly different phonologies. In the association of lexical and grammatical tonal melodies, H must be realised whereas non-associated L are deleted. Formerly associated but de-linked L however are not deleted but remain floating. The OCP is found to apply to L but not to H. H is downstepped after floating L but not after overt L. In addition, three different locations of downstep are attested which correlate with different syntactic and semantic properties of the respective constructions. In two of these downstep locations, a leftward copying process occurs in addition to a generally applicable rightward copying process so that two directions of copying occur. The thesis concludes by discussing the implications of the Ikaan findings for the wider theoretical discourse with respect to the status of the OCP, the directionality of spreading and the modelling of downstep.