The first extensive, cross-linguistic study within Distributed Morphology, this work presents a theory of the spell-out of syntactic structures as phonologically realized inflected words. Although the characteristics of well-formed words largely emerge from the interaction of forces distributed throughout the grammar (syntactic movement and the arbitrary resources of language-specific vocabulary), a residue of autonomous morphology remains, including morphosyntactic feature neutralization (Impoverishment), local re-ordering of affixes, and locally selected affix templates. The study presents a detailed synchronic and diachronic investigation of the Afroasiatic prefix conjugation from Old Akkadian to contemporary dialects of Arabic and Berber. Disjunctive ordering of morphological rules cannot simultaneously effect both position class and unique exponence effects, as in standard word-and-paradigm approaches. An original synthesis of morpheme-based and paradigm-based models is proposed in which syntactic nodes fuse or fission into their phonological signals by means of vocabulary-driven spell-out, with little or no extrinsic ordering of morphological rules. A set of feature co-occurrence restrictions or filters is provided which determines the alphabet of inflectional categories. Languages with rich inflection provide positive evidence to the learner to unlearn certain filters; otherwise, filters automatically Impoverish morphosyntactic representations, explaining the systematic absence of forms which might otherwise be constructed by freely operating word-formation rules. The filter theory of Impoverishment is exemplified with a thorough cross-linguistic study of person and number, including a comparative study of the inherent number systems of the Kiowa-Tanoan languages. The proposed theory is then tested against complex multiple-argument verbal agreement systems in Warlpiri and Nunggubuyu (Australian), Kiowa-Tanoan, and Ket (Siberia).