In Chapter 1 it was stressed that while all dental biofilms exhibit intense metabolic activity, only biofilms where a shift in metabolic activity towards an enhanced acid production over longer periods of time, will result in a net loss of mineral from the underlying tooth surface. The reflection or symptom of this is what can be detected with the naked eye and classified as the caries lesion on the tooth surface. It was pointed out that lesions may be active (if nothing changes in the oral environment, they will progress) or arrested (if nothing changes they will stay as they are). Thus, the things it is necessary to know in order to make an appropriate treatment decision are: ◆ Is a lesion present? This is detection of the lesion. ◆ Is the lesion judged to be active or arrested? This decision, adding the aspect of activity to detection, is diagnosis. ◆ Is the surface of the lesion intact or is a cavity present? If there is a cavity, can the lesion be cleaned by the patient? Diagnosis has been called a ‘mental resting place on the way to a treatment decision’. For instance, grading a lesion as active implies that the clinician considers that, if nothing is done, the demineralization will progress. Figure 3.1 is a decision tree showing how the diagnostic decision may guide the treatment. Thus, the diagnosis detects and excludes disease, assesses prognosis (considering the entire oral condition of the mouth), and forms the basis for the treatment decision. Lesions where the tooth surface is intact can be managed by the patient’s caries control measures. However, a cavity in a tooth may prevent access for the toothbrush. In addition, it may be unsightly and the tooth may be sensitive. These lesions may require restorations as a part of caries control. It is the duty of the professional to discuss with the patient whether any action is required in order to control lesion progression. Finally, the diagnosis should allow the clinical course of the disease to be monitored at subsequent visits.