In this edition of Perspectives, Reid Cornwell discusses certainty-based marking (CBM) with Tony Gardner-Medwin, professor emeritus of physiology at University College London (UCL), which adopted a simple, theoretically sound version of CBM in its medical education program. CBM has been shown to encourage thinking, reflection, improved analysis, and synthesis and to improve both the reliability and validity of students' exam data in measuring partial knowledge. However, CBM has been adopted in very few places and is sometimes regarded with skepticism. The UCL initiative has used computer technology to produce empirically convincing results. A comparison of CBM scores with conventional (correct/incorrect) scores revealed marked improvements in standard measurements of reliability. Gardner-Medwin describes the CBM system used by UCL and its philosophical, methodological, and pedagogical underpinnings, as well as the qualitative and quantitative advantages of CBM more generally.