Abawiri is a Lakes Plain language spoken in the Mamberamo River Basin in the northern lowlands of Papua, Indonesia. It is primarily spoken in a single village, Fuau, by about 550 people. The Lakes Plain languages have long been considered something of a curiosity by linguists specializing in Papuan languages. Preliminary descriptions have included reports of complex tone systems, very small consonant inventories, and ubiquitous topic-comment marking in clauses. However, the Lakes Plain languages have remained severely underdocumented, even in comparison with other Papuan language families, perhaps most of which are also underdocumented. Prior to the current work, the Abawiri language (known in the previous literature as Foau) was known only from a few short wordlists.This dissertation is a first description and analysis of the building blocks of the Abawiri language. As such, topics include segmental and tonal phonology, word classes, verbal morphology, verb phrases, clausal syntax, and clause combining constructions. Abawiri has a larger consonant inventory than other Lakes Plain languages due to a full set of labialized obstruents. The seven-vowel system includes three high front vowels. The tone system is complex and includes two tone heights that combine into eight tone patterns. Verbal morphology is primarily suffixing. Suffixes indicate a variety of tense, aspect, and mood distinctions, the most basic of which is a three-way contrast between completive, incompletive, and perfect aspect. Prefixes include directionals, polyfunctional pluractional/causative markers, and a single visual evidential. Serial verb constructions are ubiquitous in the language. The clausal syntax has a basic topic-comment structure, with an optional focused element in the comment. Noun phrases can be marked as topics; further, topic-marked clauses constitute a primary clause combining strategy in the language. The bottom-up analysis of Abawiri grammar presented here does not show clear evidence for grammatical relations. Clause combining strategies include two relative clause constructions, noun complements, purpose clauses, topical and sequential medial clauses, and clause coordination via coordinating conjunctions or juxtaposition.The description and analysis here take a functional view of language as an emergent system that evolves over time to meet the interactional needs of a community of speakers, as shaped by human cognition. Explanations are thus couched in terms of diachrony, interaction, and cognition, with reference to linguistic typology.