This chapter addresses structural aspects of language endangerment from two perspectives: the contributions that the study of endangered languages make to typology and linguistic theory, and the structural consequences of language endangerment, including the kinds of changes that can take place in the phonology, morphology and syntax of endangered languages. Typology is closely associated with the study of linguistic universals, which can be understood as the common characteristics of the world's languages, usually with the goal of providing insight into the fundamental nature of human language. Though there are various ways in which languages can become extinct, the most typical is through language shift when a language gradually comes to have fewer and fewer speakers who use it in ever fewer domains until finally no one is able to speak it in any context. This process is sometimes called language obsolescence.