[Extended abstract] This is the first detailed description of Murui (Bue variety of ‘Witoto’), a hitherto little-documented Witotoan language spoken by about 2,000 people in the Colombian and Peruvian areas of the Amazon Basin. The thesis is written following the latest theoretical requirements of modern descriptive linguistics (Dixon’s Basic Linguistic Theory). It uses extensive immersion fieldwork and participant observation as methodological techniques in the best tradition of descriptive linguistic work. Collected during several fieldtrips to the Murui communities located between the Putumayo and Caqueta rivers in Colombia (El Encanto, Tercera India, San Rafael, and San Jose) between 2013 and 2016, the linguistic data consists of an extensive corpus of texts in a variety of genres (including songs, folk tales, myths, life story narratives, narratives of traditional customs and practices, and everyday conversations), as well as field notes. In addition to the language description and analysis, the grammar also draws attention to the typological features of Murui, and sheds new light on the linguistic variation among the Witotoan languages. It is a valuable resource for further research on the linguistic affiliation of the Witotoan language family in South America. The grammar presents analyses of the phonology, morphology and syntax of the Murui language. The thesis is divided into 13 robust chapters covering specific relevant topics. These chapters are: (1) The Murui language and its speakers (sociolinguistic and genealogical information, overview of earlier work), (2) Phonology (a detailed treatment of the Murui sound system with all its complexity), (3) Word classes (an overview of open and closed word classes and their main characteristics), (4) Noun structure and classifiers (shows the central role of classifiers and repeaters in the structure of nouns and of the language in general), (5) Possession and number (on possessive constructions and the ability of nouns to be possessed), (6) Grammatical relations (the central topic in every language description), (7) Predicate structure, non-spatial, and spatial setting (a detailed presentation of predicate structure and the main categories in verbal morphology: tense, aspect, modality, evidentiality, and spatial distinctions), (8) Valency-changing mechanisms (the verbal derivational categories passive, causative, reflexive and reciprocal), (9) Adjectives and comparative constructions (on adjectives and strategies of comparison), (10) Negation (types of negation, clausal and non-clausal), (11) Commands and questions (imperatives and command strategies, types of questions), (12) Sentence types and clause linking (the structure of complex sentences, types of independent clauses), (13) Discourse organization (clause linkage, structure of narratives, focus, the influence of Spanish). Additionally, there is an elaborate list of relevant literature references, and a sample of five fully analysed illustrative texts with glosses and translation, including the origin story of the Murui people.