When first case examples presents to a dentist, a patient may have a specific complaint, be in need of routine evaluation, or arrive on referral from another health care provider. In all cases, proper diagnosis of existing problems is the essential first step in provision of appropriate oral health care. The clinician's approach to diagnosis and the need to arrive at the appropriate diagnosis are daily challenges in dental practice. This article discusses the prescriptive and descriptive theories of diagnostic reasoning using 4 case examples.