This chapter provides an introduction to the object of analysis – structural variation across a range of communicative situations in English worldwide. It presents the global spread of English in the early twenty-first century and discusses the available frameworks for understanding relations within the English language complex. The case is made that there is a disconnect between the rich level of theorizing and a lack of comprehensive empirical-linguistic accounts of differentiation in World Englishes research. The chapter then addresses the range of register variation documented in the English language. A brief discussion and definition of terms is followed by a sketch of the development in this area of research from a broad binary distinction between spoken and written language to the situationally, functionally, and structurally nuanced understanding available today. The chapter then describes how register and geography interact in structuring linguistic variation worldwide and how one may act as a confounding factor in the analysis of the other.