The biochemical definition of dental caries is reasonably understood and generally agreed upon, but there is no consensus on a clinical definition among dentists. There are many proposed diagnostic criteria of dental caries in the dental literature. The recently developed International Caries Detection and Assessment System (ICDAS II) has been constructed to allow data comparison between studies. It can be used in epidemiological studies, public health research, clinical research, clinical practice and dental education. A good study evaluating a caries detection method should contain information on caries prevalence of the study sample and other measures, including sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV). It is noteworthy that measuring sensitivity and specificity provides no quantitative information on how likely a tested tooth is to be carious because the true caries status of the tooth is not known in a clinical situation. Moreover, the study design for caries detection should address the interpretation of predictive values because PPV and NPV are affected by the caries prevalence. The study design should also measure patient-oriented outcomes, address allocation concealment and avoid lead-time bias to generate valid and clinically relevant studies. Prudent evaluation of caries detection methods is the standard of care. This paper reviews current diagnostic criteria for caries detection and discusses proper ways to evaluate new diagnostic methods.