This paper outlines a theoretical approach to multilingualism in Europe that draws on recent work in realist social theory (RST). It sets out the key concepts of culture, structure and agency, and develops a view of language as a cultural emergent property - that is, as an outcome of the dynamic interplay between these ontological components of the world. The paper provides an indication of the relevance of these ideas for language policy and planning in Europe and explores the methodological implications of our particular account of RST. It also provides an introduction to the subsequent papers, which discuss empirical examples of various European contexts where multilingualism is a feature.