This book is a linguistic study of Jahai, a language belonging to the Northern Aslian subgroup of the Aslian branch of the Mon-Khmer language family. The language is spoken by groups of foragers in the mountain rainforests of northern Peninsular Malaysia and southernmost Thailand, its total number of speakers estimated at around 1,000. This study describes the grammar of Jahai, including its phonology, processes of word formation, word classes, and syntax. It also includes a word-list. While primarily aimed at linguistic description, the study makes use of suitable theoretical models for the analysis of linguistic features. In particular, models of Prosodic and Template Morphology are employed to describe the language's intricate processes of affixation. Typological comparisons are made at times, especially with other Aslian languages. The study is intended to expand our knowledge of the understudied Aslian languages. It is also intended to contribute to Mon-Khmer and Southeast Asian language studies in general, and, hopefully, also to a wider linguistic context. Furthermore, it may serve as a practical source of linguistic information for researchers and others working among the Northern Aslian speech communities.