The key-features (KFs) approach to assessment was initially proposed during the First Cambridge Conference on Medical Education in 1984 as a more efficient and effective means of assessing clinical decision-making skills. Over three decades later, we conducted a comprehensive, systematic review of the validity evidence gathered since then. The evidence was compiled according to the "Standards for Educational and Psychological Testing"'s five sources of validity evidence, namely, Content, Response process, Internal structure, Relations to other variables, and Consequences, to which we added two other types related to Cost-feasibility and Acceptability. Of the 457 publications that referred to the KFs approach between 1984 and October 2017, 164 are cited here; the remaining 293 were either redundant or the authors simply mentioned the KFs concept in relation to their work. While one set of articles reported meeting the validity standards, another set examined KFs test development choices and score interpretation. The accumulated validity evidence for the KFs approach since its inception supports the decision-making construct measured and its use to assess clinical decision-making skills at all levels of training and practice and with various types of exam formats. Recognizing that gathering validity evidence is an ongoing process, areas with limited evidence, such as item factor analyses or consequences of testing, are identified as well as new topics needing further clarification, such as the use of the KFs approach for formative assessment and its place within a program of assessment.