Revalidation was launched in the UK to provide assurances to the public that doctors are up to date and fit to practice. Appraisal is a fundamental component of revalidation. Approximately 150 000 doctors are appraised annually, costing an estimated £97 million over 10 years. There is little understanding of the theory of how and why appraisal is supposed to produce its effects. A realist review of the literature was utilised to explore these issues, as they generate context-mechanism-outcome (CMO) configurations, resulting in the creation of theories of how and why appraisal of doctors produces its effects.