In general terms, this monograph discusses the relation between empirical evidence and explanation in historical linguistics. The main focus is on language contacts during the Great Migrations of the first millennium AD, in the first place on Slavic and Celtic, with an additional excursus into the time of the Indo-European migrations. It is shown how various kinds of evidence - linguistic, cultural, archaeological and historiographical - can be combined in a complex inductive-deductive process of analysis, finally arriving at a relatively complete explanation of the investigated phenomena.