Languages with rich inflectional systems frequently exhibit contrasting inflection classes, each of which is associated with its own particular inventory of inflectional markings; examples are the Latin declension classes and conjugation classes. In Chapter 6 (‘Inflection classes’), I discuss the properties of such classes, distinguishing between global inflection classes (which determine full paradigms) and segregated inflection classes (which determine specific subparadigms). I address the important question of what inflection classes are classes of; as I show, the standard assumption that they are classes of lexemes is difficult to reconcile with the phenomenon of heteroclisis (Stump 2006), which instead favors the assumption that they are classes of stems. While inflection classes are often distinguished by different inventories of affixes, they are very frequently distinguished by their patterns of stem formation and stem alternation.