This book is the first comprehensive grammatical description of Uchumataqu, the language of the Uru of Lake Titicaca in north-western Bolivia. Uchumataqu forms part of the isolated language family Uru-Chipaya but has been influenced to differing degrees by Aymara, Quechua, and Spanish. The Uchumataqu language became extinct around 1950. Although several researchers had documented the language during the first half of the twentieth century, much of the material remained unstudied. This book is the first to take into consideration every previous study of the Uchumataqu language. The grammatical description is based on former publications and archive material and seeks to describe Uchumataqu as comprehensively as possible. It includes a description of the phonological system of Uchumataqu as well as a presentation of its morphological processes. The nominal and verbal systems are discussed in detail. Particular attention is paid to the complex person-marking system of Uchumataqu, of which person-marking clitics are a vital part that distinguishes Uchumataqu from the neighbouring Aymara and Quechua language. Another important issue are nominalisation and subordination strategies as well as adjectives which form a word class of its own. The relationship of Uchumataqu with the surrounding Aymara and Quechua language, and particularly the way in which influence on Uchumataqu was exerted, are described in detail. The appendices contain a transcription of the afore unpublished manuscripts of Max Uhle and Walter Lehmann on Uchumataqu as well as a comparative and diachronic dictionary. This book is aimed at linguists from all disciplines but is of equal interest to anthropologists, Americanists, historical linguists, typologists, and linguists with a special interest in Andean studies. It is not only an important contribution to the study of Andean languages and their interrelationship, but also an account for the descendants of the last Uchumataqu speakers of their lost language.