This is a comprehensive reference grammar of Tariana, an endangered Arawak language from a remote region in the northwest Amazonian jungle. Its speakers traditionally marry someone speaking a different language, and as a result most people are fluent in five or six languages. Because of this rampant multilingualism, Tariana combines a number of features inherited from the protolanguage with properties diffused from neighbouring but unrelated Tucanoan languages. Typologically unusual features of the language include: an array of classifiers independent of genders, complex serial verbs, case marking depending on the topicality of a noun, and double marking of case and of number. Tariana has obligatory evidentiality: every sentence contains a special element indicating whether the information was seen, heard, or inferred by the speaker, or whether the speaker acquired it from somebody else. This grammar will be a valuable source-book for linguists and others interested in natural languages.