This thesis examines the inventory of morphemes Sumerian utilizes to denote modal notions on the verb. Sumerian is an agglutinative linguistic isolate that was spoken in southern Mesopotamia from at least the fourth millennium BCE to sometime early in the second millennium BCE. With respect to its morphology, the Sumerian language utilizes a set of affixes that can occupy certain slots in the agglutinative verbal prefix chain to code modality – a notional category that expresses a speaker’s stance on utterances relative to reality and unreality. Understanding the ways modal notions are coded in any language is crucial as they are linguistic means to express high degrees of nuance. As such, the study of modality in ancient languages such as Sumerian will pave the way for an increasingly sophisticated understanding of the texts, peoples, languages, and cultures that are millennia removed from our own.This study is a corpus-based inquiry that follows the guidelines of Functional Discourse Grammar and implements an onomasiological methodology. In CHAPTER ONE, I provide an overview of previous Sumerological scholarship and a general sketch of this dissertation’s method, theory, and layout. CHAPTER TWO includes an overview of modality as a linguistic category and the basics of its expression in Sumerian. CHAPTER THREE is the first argumentative chapter. In this chapter, I cover how all epistemic modal notions are marked morphologically on the verb. The ways in which all deontic modal notions are marked morphologically on the verb are outlined in CHAPTER FOUR. The various morphological manifestations of evidential modality on Sumerian verbs are covered in CHAPTER FIVE. All modal phenomena that were unable to be included in a dedicated content chapter are discussed in 6 CHAPTER SIX. In CHAPTER SEVEN, I organize all my findings by form, not function, to help scholars in search of a more traditional presentation of data. CHAPTER EIGHT includes my concluding remarks. APPENDICES outlining the corpora and remaining uncited Asseveratives, an INDEX of cited Sumerian verbs, and a standard BIBLIOGRAPHY conclude the dissertation.