This chapter lays out the theoretical devices on which the subsequent analysis builds. The first section introduces the phonological architecture used in the book, and in particular the distinction between the phonological, phonetic-phonological, and phonetic levels of representation, which underpin the notion of the life cycle that is central to the book’s argument. The second section recaps current views on the mechanisms of language contact and the role that phonological patterns can play under different contact scenarios. This is followed by a discussion of areal effects in phonology generally and some case studies beyond northern Europe, which exemplify various possibilities for recovering the history of contact from attested phonological patterns. The third section discusses the mechanisms of phonological convergence and the possible uses of sound patterns in diagnosing language contact in the past. Finally, the fourth section lays out the theory of the life cycle of phonological processes and introduces key related notions such as rule scattering and rule generalization, and lays out a proposal for how the life cycle can be used in examining language contact in the past.