A possible consequence of light absorption as the interaction of electromagnetic radiation with molecules of the tooth's hard substances is, apart from the emission of heat, fluorescence. It was demonstrated that the emission spectra of enamel, dentin, and caries look alike upon excitation with red light; however, fluorescence increases with the process of carious destruction. Based on that, the DIAGNOdent System was developed, which simultaneously injects red light into the tooth surface and detects the resulting fluorescence. Due to this design, clinical application is currently limited to accessible occlusal and smooth surfaces. For occlusal caries detection, it was shown that validity and reproducibility could be optimized over classical diagnostic tests for deep dentin lesions with seemingly intact surfaces. The detection of root caries does not play a relevant role in the directly accessible areas, but estimation of the lesion's activity and progression rate is a prerequisite for differentiated lesion management Excellent reproducibility of the laser fluorescence measurements was confirmed for this scenario. Lesions with a soft surface texture show significantly higher fluorescence values than leathery or hard lesions, and values also increase with increasing cavity depth. DIAGNOdent offers the potential to improve follow-ups and estimation of the prognosis and to support clinical management of primary carious lesions.