This article introduces the concept of a key feature and describes its function as the cornerstone of key-feature problems, a new problem format for the written assessment of clinical decision-making skills of medical trainees and practitioners. The rationale for using this problem format and the steps in problem and examination development--including issues of scoring and standard setting--are described. A key feature is defined as a critical step in the resolution of a clinical problem, and a key-feature problem consists of a clinical case scenario followed by questions that focus on only those critical steps. The questions can be presented to require examinees either to write in their responses or to select them from a list of options. For each question, examines can be instructed to supply or select whatever number of responses is appropriate to the clinical task being tested, and answer keys can comprise one or several responses. This problem format, with its focus on only the critical steps in problem resolution, and with its flexibility in question format and scoring keys, effectively addresses the psychometric considerations of content validity and test score reliability, and accommodates the complexity and configurations of actions often required in the resolution of clinical problems.