The script concordance test (SCT) is used in health professions education to assess a specific facet of clinical reasoning competence: the ability to interpret medical information under conditions of uncertainty. Grounded in established theoretical models of knowledge organization and clinical reasoning, the SCT has three key design features: (1) respondents are faced with ill-defined clinical situations and must choose between several realistic options; (2) the response format reflects the way information is processed in challenging problem-solving situations; and (3) scoring takes into account the variability of responses of experts to clinical situations. SCT scores are meant to reflect how closely respondents' ability to interpret clinical data compares with that of experienced clinicians in a given knowledge domain. A substantial body of research supports the SCT's construct validity, reliability, and feasibility across a variety of health science disciplines, and across the spectrum of health professions education from pre-clinical training to continuing professional development. In practice, its performance as an assessment tool depends on careful item development and diligent panel selection. This guide, intended as a primer for the uninitiated in SCT, will cover the basic tenets, theoretical underpinnings, and construction principles governing script concordance testing.