Book excerpt: Language contact - the linguistic and social outcomes of two or more languages coming into contact with each other - has been pervasive in human history. However, where histories of language contact are comparable, experiences of migrant populations have been only similar, not identical. Given this, how does language contact work? With contributions from an international team of scholars, this Handbook - the first in a two-volume set - delves into this question from multiple perspectives and provides state-of-the-art research on population movement and language contact and change. It begins with an overview of how language contact as a research area has evolved since the late 19th century. The chapters then cover various processes and theoretical issues associated with population movement and language contact worldwide. It is essential reading for anybody interested in the dynamics of social interactions in diverse contact settings and how the changing ecologies influence the linguistic outcomes.