Noun incorporation (NI) is a grammatical construction where a nominal, usually bearing the semantic role of an object, has been incorporated into a verb to form a complex verb or predicate. Traditionally, incorporation was considered to be a word formation process, similar to compounding or cliticization. The fact that a syntactic entity (object) was entering into the lexical process of word formation was theoretically problematic, leading to many debates about the true nature of NI as a lexical or syntactic process. The analytic complexity of NI is compounded by the clear connections between NI and other processes such as possessor raising, applicatives, and classification systems and by its relation with case, agreement, and transitivity. In some cases, it was noted that no morpho-phonological incorporation is discernable beyond perhaps adjacency and a reduced left periphery for the noun. Such cases were termed pseudo noun incorporation, as they exhibit many properties of NI, minus any actual morpho-phonological incorporation. On the semantic side, it was noted that NI often correlates with a particular interpretation in which the noun is less referential and the predicate is more general. This led semanticists to group together all phenomena with similar semantics, whether or not they involve morpho-phonological incorporation. The role of cases of morpho-phonological NI that do not exhibit this characteristic semantics, i.e., where the incorporated nominal can be referential and the action is not general, remains a matter of debate. The interplay of phonology, morphology, syntax, and semantics that is found in NI, as well as its lexical overtones, has resulted in a wide range of analyses at all levels of the grammar. What all NI constructions share is that according to various diagnostics, a thematic element, usually correlating with an internal argument, functions to a lesser extent as an independent argument and instead acts as part of a predicate. In addition to cases of incorporation between verbs and internal arguments, there are also some cases of incorporation of subjects and adverbs, which remain less well understood.