Paradigm Function Morphology (PFM) is an evolving approach to modeling morphological systems in a precise and enlightening way. The fundamental insight of PFM is that words have both content and form and that in the context of an appropriately organized lexicon, a language’s morphology deduces a complex word’s form from its content. PFM is therefore a realizational theory: a language’s grammar and lexicon are assumed to provide a precise characterization of a word’s content, from which the language’s morphology then projects the corresponding form. Morphemes per se have no role in this theory; by contrast, paradigms have the essential role of defining the content that is realized by a language’s morphology. At the core of PFM is the notion of a paradigm function, a formal representation of the relation between a word’s content and its form; the definition of a language’s paradigm function is therefore the definition of its inflectional morphology. Recent elaborations of this idea assume a distinction between content paradigms and form paradigms, which makes it possible to account for a fact that is otherwise irreconcilable with current morphological theory—the fact that the set of morphosyntactic properties that determines a word’s syntax and semantics often differs from the set of properties (some of them morphomic) that determines a word’s inflectional form. Another recent innovation is the assumption that affixes and rules of morphology may be complex in the sense that they may be factored into smaller affixes and rules; the evidence favoring this assumption is manifold.