Affixation is the morphological process that consists of adding an affix (i.e., a bound morpheme) to a morphological base. It is cross-linguistically the most common process that human languages use to derive new lexemes (derivational affixation) or to adapt a word’s form to its morphosyntactic context (inflectional affixation). Suffixes (i.e., bound morphemes following the base) and prefixes (i.e., bound morphemes preceding the base) are the most common affixes, with suffixation being more frequently recorded in the world’s languages than prefixation. Minor types of affixation include circumfixation and infixation. Conversion and back-formation are related derivational processes that do not make use of affixation. Many studies have concentrated on the need to differentiate derivation from inflection, but these morphological processes are probably best described as two end points of a cline. Prototypically, derivation is used to change a word’s category (part of speech) and involves a semantic change. A word’s inflectional distinctions make up its paradigm, which amounts to the different morphological forms that correlate with different morphosyntactic functions. Form-function mapping in (derivational and inflectional) affixation is a key issue in current research on affixation. Many deviations from the canonical One Form-One Meaning principle can be observed in the field of affixation. From a diachronic point of view, it has been demonstrated that affixes often derive from free lexemes by grammaticalization, with affixoids being recognized as an intermediate step on this cline. More controversial, but still attested, is the opposite change whereby affixes and affixoids develop into free morphemes through a process of degrammaticalization.