Nonconcatenative morphology refers to a type of word formation involving modification of the internal structure of a word. This study includes a survey and detailed examination of the historical processes that have created and modified the nonconcatenative morphological alternations in the Semitic language family and discussion of the consequences these processes have for our understanding of morphological structure more generally.

This thesis argues that the developments and resulting patterns of Semitic morphology can be accounted for by reference to a small set of basic mechanisms of change. The most fundamental mechanism is reinterpretation, in which a listener interprets an input differently from that intended by a speaker. The frequency of a particular change is dependent on the likelihood of a reinterpretation due to inherent ambiguities and biases introduced by general human cognition, the physics and physiology of speech and contact between languages. Three main processes result in the creation or disruption of nonconcatenative morphology. The first and perhaps most important is the morphologization of previously phonological alternations. This includes alternations related to the long-distance influence of a vowel or consonant and those occasioned by the prosodic structure of a word, particularly stress placement. The other two processes are analogy and the reinterpretation of syntactic structures as morphological ones.

Nonconcatenative alternations are so prevalent in the Semitic languages that words can be analyzed as consisting of a "root" made up of consonants indicating the basic meaning and "patterns" that provide a more specific meaning or syntactic function. While the Semitic roots and patterns certainly have a psychological reality, they do not play a role in every domain. Unlike other morphological constituents, the patterns are not used in processes of analogical leveling nor do they appear to inhibit changes which make alternations more opaque. The historical processes that affect the morphology proceed largely without reference or regard to the existing roots and patterns.