Overabundance is the situation in which two (or more) inflectional forms are available to realize the same cell in the inflectional paradigm of a lexeme (i.e., to express the meaning arising from the combination of the lexical meaning of the lexeme and the morphosyntactic and morphosemantic feature values that define the cell). An example from English is dreamed / dreamt, both of which realize ‘dream.pst’, the past tense of the verb dream. The forms that realize the same cell are called cell mates. Certain cell mates can be used interchangeably in the same context, even by the same speaker, with no difference in style and meaning; other cell mates are subject to various kinds of conditions, so that different forms are used in different styles, registers, social or geographic dialects, or in different semantic, syntactic or pragmatic contexts. Cell mates by definition are formally different in some respect: they may display different stems (e.g., Italian sepol-to / seppelli-to ‘bury-pst.ptcp’), or different endings (e.g., Czech jazyk-u / jazyk-a ‘language-gen.sg’), they may be built according to different means (e.g., English more choosy / choosier ‘choosy.comp’, where the first cell mate is periphrastic and the second one synthetic), or they may differ in various other ways. Overabundance can be limited to a specific cell of a specific lexeme, or it can occur systematically in certain cells of certain lexemes or of all lexemes in a given word class and language (e.g., all Spanish verbs have two ways of realizing all the forms of the imperfect subjunctive). Most linguists assume that overabundance can exist only as a transitional stage during diachronic change, and that any single speaker only uses one of the cell mates available in a community’s repertoire; besides, many assume that cell mates always differ according to geo-socio-stylistic conditions, or in meaning. However, corpus based studies of specific instances of overabundance have shown that there are cases of truly interchangeable cell mates, that a single speaker can use different cell mates even within the same utterance, and that some instances of overabundance are stably attested for centuries. Language standardization often aims at eliminating overabundance, but low frequency forms may escape elimination and remain in usage. Many principles assumed to regulate language acquisition (e.g., Clark’s Principle of Contrast) ban overabundance; however, forms acquired later than in the early stages of language acquisition, sometimes only with schooling, may escape this ban. Even principles of grammar, such as Blocking, or Pāṇini’s principle, appear to entail the impossibility of having synonymous cell mates. However, much depends on the exact formulation of these principles; and the existence of cell mates can be reconciled with certain versions of them and has been acknowledged in much recent work in theoretical morphology.