Objective: The purpose of this study was to compare in vivo the efficacy of DIAGNOdent measurements in the diagnosis of occlusal caries with conventional diagnostic methods (visual inspection and radiographic methods). Methodology: A total of 187 molar teeth in 161 individuals (87 female, 74 male) between the ages of 18 and 25, who were admitted to the Ondokuz Mayis University Faculty of Dentistry, Department of Restorative Dentistry, were included in this study. The study was evaluated by Ondokuz Mayis University Faculty of Medicine Ethical Committee and approved as ethically applicable. In the diagnostic phase of the study, all subjects' teeth were evaluated by two observers sequentially by visual inspection, radiography, and using DIAGNOdent. Carious lesions on the occlusal surfaces of molar teeth, which were evaluated by visual inspection, radiography and DIAGNOdent, were removed by using rotational cutting devices. Resulting cavities following caries removal were visually evaluated, and the level of caries was determined. Results: Statistically significant differences were found when the visual inspection results, radiographic evaluation results, and DIAGNOdent evaluation results of two observers were compared for cavity depths. Conclusion: Although DIAGNOdent is an important diagnostic device, particularly for diagnosing hidden carious lesions in the dentine layer, it should be used along with visual examination in clinical practice, as DIAGNOdent is not superior to visual examination in detecting early stages of carious lesions.