This entry describes the state of knowledge on how inflection systems are structured in terms of classes. After defining inflection classes as sets of lexemes that share some inflectional behaviour, various points of variation of how inflection classes are conceptualized in the literature are discussed first. Next, the entry shows how inflection classes tend to group lexemes that are otherwise similar in their linguistic properties, for reasons relatable to their diachronic origins. This is followed by the study of the structure of inflection class systems, arguing that it is much richer and intricate than traditional accounts suggest. The entry closes by outlining research on the limits on inflectional variability.