Oneida is an endangered Iroquoian language spoken fluently by fewer than 250 people. This is the first comprehensive dictionary of the Oneida language as used in Ontario, where most of the surviving speakers reside. The dictionary contains both Oneida-English and English-Oneida sections. The Oneida-English portion includes some 6000 entries, presenting lexical bases, particles, and grammatical morphemes. Each entry for a base shows several forms; illustrates inflection, meaning, and use; and gives details regarding pronunciation and cultural significance. The English-Oneida entries direct the reader to the relevant base in the Oneida-English section, where technical information is provided. Completing the volume is a set of appendices that organizes Oneida words into thematic categories. The Iroquoian languages have an unusually complex word structure, in which lexical bases are surrounded by layers of prefixes and suffixes. This dictionary presents and explains that structure in the clearest possible terms. A work of enormous precision and care, it incorporates many innovative ideas and shows a deep understanding of the nature of the Oneida language.