Language contact studies and historical linguistics, i.e. the study of language change, are subfields of linguistics that have long been recognized as being mutually relevant. This chapter explores this relationship along two dimensions: first, with regard to the fields of study themselves, and second, and perhaps more importantly, with regard to those aspects of language contact and of influence external to a given linguistic system that are particularly relevant to understanding the basic subject matter of historical linguistics, i.e. what happens to languages as they pass through time. In terms of the fields of study, an overview of the historiography of the distinction between internally motivated and externally motivated change is offered. This survey is followed by a discussion of several case studies, in which language contact serves as an actuator of change as well as some in which it is an inhibitor of change. Finally, the interaction of language contact with another key issue in historical linguistics, namely language genealogy, is discussed, along with a consideration of the naturalness and pervasiveness of language contact.