This thesis examines the different bases and meanings of ‘equality and non-discrimination’ in international law. Its main inquiry is to assess the relevant principles, methods, and tools employed by selected treaty bodies to shape the conceptions of ‘substantive equality’ in human rights law. It will be submitted that ‘substantive equality’ is understood very differently due to the doctrinal and interpretive choices and decisions these bodies take in their adjudication purposes and processes. These choices can, however, be evaluated and challenged by the use of ‘nexus equality’ in their treaty context. The author concludes that the treaty body which pays due regard to ‘nexus equality” enables its practice to be critical yet reflexive of its doctrinal choices and as such provides the most advanced understanding of ‘substantive equality.’