This thesis describes and analyzes the grammar of regressive nasal harmony in Paraguayan Guarani, a Tupi-Guarani language spoken by millions in Paraguay. Based on data from original fieldwork conducted in Coronel Oviedo, Paraguay, this thesis makes two empirical contributions and analytical claims in the study of Guarani nasal harmony. First, I show that Guarani suffixes display independence with regards to the distribution of the nasal-oral contrast, otherwise observed in roots and prefixes, due to the cyclic nature of suffixes combined with the limited domain of positional effects of nasality. I analyze this fact in a constraint-based framework by proposing output-to-output correspondence constraints as well as higher ranked requirements for faithfulness in nasality in the domain of suffixes. Finally, I argue that Guarani’s nasal-oral stops are, contrary to previous literature, phonologically full nasal segments that are postoralized in the presence of oral vowels. I present an analysis that unifies such claim with the general analysis of regressive nasalization, and briefly show that an analysis of progressive nasalization as phonologically conditioned allomorphy is compatible with both arguments proposed in this work.