Clinical decision making in dentistry involves some uncertainty and error, but in the absence of good clinical research it is often more of an art than a science. This article demonstrates a way in which clinicians can consistently and effectively base the diagnostic process on scientific evidence. The authors examine the selection and utility of supplemental diagnostic tests, describe the properties of these tests, and explain how test results can be used to augment clinical opinion. To aid the dental clinician in the decision making process, a step-by-step strategy is presented for the detection of squamous cell carcinoma using toluidine blue dye.