This dissertation studies the role purity (dag pa) plays in 14th-century Longchen Rabjam’s (klong chen rab ‘byams) presentation of the Great Completeness (rdzogs chen). In service of this aim, I rigorously consider the semantic field of purity and its impact on our contemporary understanding of the Great Completeness. I specifically consider four major themes related to purity: space, non-change, luminosity, and effort. In doing so, my analysis builds on existent scholarship on the Great Completeness, highlighting how understanding Longchen Rabjam’s usage of purity elucidates often misunderstood elements of this contemplative system. Taken comprehensively, the semantic field of purity consists of a broad and remarkably diverse spectrum of attributes, each of which are employed by Longchenpa in service of describing the Great Completeness. In this way, I contend that purity is a vital lens for understanding Longchenpa’s presentation of the Great Completeness and its critical shift in emphasis from the ethical to the epistemological.