This thesis explores cross-linguistic variation in nasal harmony. The goal is to unify our understanding of nasal harmony so that patterns across languages conform to one basic character and to examine the wider implications of this account for phonological theory. The analysis builds on generalizations from a comprehensive survey documenting variation in three descriptive sets of segments in nasal harmony: targets, which become nasalized, blockers, which remain oral and block spreading, and transparent segments, which remain oral but do not block. The typological generalizations established by this study provide strong support for a unified view of nasal harmony in which variation is limited in a hierarchical fashion. To capture cross-lin...