Reliability of clinical diagnosis is essential for good clinical decision making as well as productive clinical research. The current review emphasizes the distinction between a disorder and a diagnosis and between validity and reliability of diagnoses, and the relationships that exist between them. What is crucial is that reliable diagnoses are essential to establishing valid diagnoses. The present review discusses the theoretical background underlying the evaluation of diagnoses, possible designs of reliability studies, estimation of the reliability coefficient, the standards for assessment of reliability, and strategies for improving reliability without compromising validity.