Gumuz is a Nilo-Saharan dialect cluster spoken in the river valleys of northwestern Ethiopia and the southeastern part of the Republic of the Sudan. There are approximately 200,000 speakers, the majority of which reside in Ethiopia. This study is a phonological and grammatical analysis of two main dialects/languages: Northern Gumuz and Southern Gumuz. The study provides an overview of the Gumuz people and culture, including historical accounts of the language(s) and migration patterns. Most major aspects of the language are described and analyzed in detail: phonology, nouns, pronouns, demonstratives and other noun phrase constituents, verbs and verbal morphology, noun incorporation, verbal classifiers, noun categorization, basic clauses, and subordinate clauses. Northern and Southern Gumuz varieties are contrasted throughout. Gumuz tone has two levels, High and Low, with tonal downstep of High. The tonal melody on bound pronominals on verbs indicates transitivity. Nouns are divided into two basic types: relational and absolute. Relational nouns have an inherent relationship with another nominal element, either within a noun-noun compound or with a (historical) possessive affix. Two sets of relational nouns --attributive and relator nouns-- obligatorily take an inherent possession suffix if not in a compound. Gumuz has two noun-noun constructions: the Associative Construction and the Attributive Construction. The first is left-headed with `noun of noun' semantics. The second is right-headed with the initial noun expressing an inherent quality of the second. Certain body part terms have grammaticalized as a variety of other morphosyntactic categories, in particular as relator nouns, verbal classifiers, and class morphemes, the final two of which are noun categorization devices. Many of these same body part terms can be incorporated into the verb or form part of lexicalized verb-noun compounds. Deverbal nominalizations with /ma-/ are found throughout the language structures. These /ma-/ nominalizations serve as both subject and object complements. They are also commonly found in other subordinate clauses such as relative and adverbial clauses. Purpose clauses are formed with the dative preposition plus a /ma-/ nominalization. Finite purpose clauses take pronominal inflection and have further grammaticalized as future tense main clause verbs in Southern Gumuz.