This book presents a comprehensive survey of historically attested relative clause constructions from a diachronic typological perspective. Systematic integration of historical data and a typological approach demonstrates how typology and historical linguistics can each benefit from attention to the other. The diachronic behaviour of relative clauses is mapped across a broad range of genetically and geographically diverse languages. Central to the discussion is the strength of evidence for what have previously been claimed to be 'natural' or even 'universal' pathways of change. While many features of relative clause constructions are found to be remarkably stable over long periods of time, it is shown that language contact seems to be the crucial factor that does trigger change when it occurs. These results point to the importance of incorporating the effects of language contact into models of language change rather than viewing contact situations as exceptional. The findings of this study have implications for the definition of relative clauses, their syntactic structures and the relationships between the different 'subtypes' of this construction, as well as offering new directions for the integration of typological and historical linguistic research.