Previous research on Sumi, a Tibeto-Burman language spoken in the extreme northeast of India, has found it to have three lexical tones. However, the few phonological studies of Sumi have focused mainly on its segmental phonology and have failed to provide any substantial account of the tone system. This thesis addresses the issue by providing the first comprehensive description of tone in this language. In addition to confirming three contrastive tones, this study also presents the first acoustic phonetic analysis of Sumi, looking at the phonetic realisation of these tones and the effects of segmental perturbations on tone realisation. The first autosegmental representation of Sumi tone is offered, allowing us to account for tonal phenomena such as the assignment of surface tones to prefixes that appear to be lexically unspecified for tone. Finally, this investigation presents the first account of morphologically conditioned tone variation in Sumi, finding regular paradigmatic shifts in the tone on verb roots that undergo nominalisation. / The thesis also offers a cross-linguistic comparison of the tone system of Sumi with that of other closely related Kuki-Chin-Naga languages and some preliminary observations of the historical origin and development of tone in these languages are made. This is accompanied by a typological comparison of these languages with other Tibeto-Burman languages, which shows that although these languages are spoken in what has been termed the ‘Indosphere’, their tone systems are similar to those of languages spoken further to the east in the ‘Sinosphere’. Finally, a more global typological comparison of Sumi with ‘African’ and ‘East Asian’ tone languages demonstrates that Sumi displays features typically associated with both these language ‘types’. This finding suggests the need to re-evaluate this traditional dichotomy of tone systems, and the need to consider morphological structure in typologies of tone.