It is hypothesized that occlusal lesions are initiated on the fissure walls and can therefore be obscured by sound superficial tissue. Additionally, there is evidence that one effect of regular use of fluorides is greater opacity of enamel, which may obscure underlying lesions in dentin, the so-called 'hidden lesions'. Dental radiographs are inadequate for detecting decay in the occlusal surfaces until the lesion is well advanced through the enamel and into the dentin. The clinician relies on visual observation of texture and discoloration, clinical judgment based upon experience, and on tactile sense by probing with an explorer. An objective detection method to complement the traditional visual assessment is used by the clinician for arriving at clinical decisions on the management of the carious lesion: whether invasive therapy or a more conservative, noninvasive approach. Objective and reliable longitudinal monitoring of the lesion's response to preventive measures allow the selection of an appropriate therapy before the lesion progresses to the stage where invasive treatment is required. This paper discusses the problem of the lack of appropriate clinical methods for the detection and quantification of carious lesions. A few commercially available methods are described (the quantitative light-induced fluorescence method, the DIAGNOdent device, and electrical caries monitor) and some new techniques mentioned.