Noun incorporation is perhaps the most nearly syntactic of all morphological processes. Examination of the phenomenon across a large number of geographically and genetically diverse languages indicates that, where syntax and morphology diverge, incorporation is a solidly morphological device that derives lexical items, not sentences. It is used for four different but related purposes; these fall into an implicational hierarchy which in turn suggests a path along which incorporation develops historically. Differences in its productivity from language to language show that this development may be arrested at any point—resulting either in the eventual disappearance of the process, or in its resurgence as a productive system of affixation.