This chapter highlights the linguistic study of Native American signed language varieties, which are broadly referred to as American Indian Sign Language (AISL). It describes how indigenous sign language serves as an alternative to spoken language, how it is acquired as a first or second language, and how it is used both among deaf and hearing tribal members and internationally as a type of signed lingua franca. It discusses the first fieldwork carried out in over fifty years to focus on the linguistic status of AISL, which is considered an endangered language variety but is still used and learned natively by some members of various Indian nations across Canada and the United States (e.g. Assiniboine, Blackfeet/Blackfoot, Cherokee, Crow, Northern Cheyenne, Nakoda/Lakȟóta, and Mandan-Hidatsa). The chapter also addresses questions of language contact and spread, including code-switching and lexical borrowing, as well as historical linguistic questions.