The search for an adequate conceptualisation of the Uniform Mark Scale (UMS) is a challenging one and it is clear that there is a need to broaden current discussions of the issues involved. This article marks an attempt to demystify the UMS; its conception and operation. Although the article assumes a basic appreciation of the terminology and processes associated with the examination system, it explicates through a number of case study scenarios, the contexts in which it is appropriate to employ UMS, describes any necessary computations arising from different specifications and assessment scenarios, and addresses some of the potential challenges posed by the calculation of grades for unitised specifications. A specification here refers to a comprehensive description of a qualification and includes both obligatory and optional features: content, and any performance requirements. If a specification is unitised, the constituent units can be separately delivered, assessed and certificated. Having a clear and well-articulated position on the underlying theory of UMS is necessary to demonstrate transparency with regard to the estimation of aggregate performance on unitised assessments and to support any claims we wish to make about the reporting process. It is hoped that the issues addressed here will make a positive contribution to the widening nature of the UMS debate (both within and beyond Cambridge Assessment) more generally, and of the understanding, operation and employment of UMS, in particular.