The book explores inflectional morphology from a micro-typological perspective in order to determine crucial factors in language change. It is shown that Bybee’s synchronically-defined relevance principle also enables diachronic predictions of the direction of language change, i.e. the strengthening of relevant inflectional categories (ASPECT, VOICE, TENSE, MOOD) vs. weakening of less relevant categories (NUMBER, PERSON). Exceptions may be explained by the lack of phonological input (i-umlaut in Swedish and Dutch) or due to divergent category frequency (preterit): Category frequency crucially affects restructuring and simplification in the ablaut system of strong verbs: Swedish retains an aspectual distinction within the past tenses, leading to a high category frequency of both the preterit and the perfect. As a consequence, ablaut patterns that keep the two past tenses distinct are preferred (ABC: finna ‘find’ – fann – funnit or ABA: skriva ‘write’ – skrev – skrivit). In contrast, the preferred ablaut pattern in German is ABB (schreiben – schrieb – geschrieben), which is in line with the preference for the perfect and the concomitant loss of the preterit in spoken language.