This book provides a typologically-informed grammatical analysis of Yalaku, a language spoken by about 900 people in three villages in East Sepik Province, Papua New Guinea. The language has many striking features, including an 'impeditive' modality to describe something hard to do; a five-way demonstrative system; directional markers with demonstratives and with verbs; and a masculine/feminine gender split whereby the former is associated with elongated shape and large size and the latter with small size and roundish shape. The interaction of phenomena shared with related languages and those that evolved via language contact with Tok Pisin, the national language of Papua New Guinea, and Kwoma, spoken by close neighbours, add to the complex tapestry of the language. After an introductory account of the language, its speakers, and the correlations between subsistence patterns and language use, the chapters explore phonetics, phonology, grammatical and phonological word, word classes, gender, number, case marking, dynamic and stative verbs, modality, mood, imperatives and commands, serial verb constructions and complex predicates, clause chaining, and grammaticalization processes. Throughout, Alexandra Aikhenvald provides an up-to-date assessment of correlations between language, its ecology, and the physical, social, and cultural environment in flux in one of the most linguistically diverse regions of the world, using rich illustrative material from the author's own fieldwork.