Following the consideration of several recent systematic and other reviews, there is a growing professional and scientific consensus that caries measurement methodology in caries clinical trials (CCT) should be updated to reflect progress made elsewhere in cariology. In this paper, therefore, "modern" means accepted in contemporary dental research and dental practice on the basis of sound research evidence--not necessarily new or requiring the use of new technology. Caries measurement should be seen in the context of the objectives of modern clinical caries management and the continuum of disease states, ranging from sub-surface carious changes through to more advanced lesions. Measurement concepts can be applied to at least three levels: the tooth surface, the individual, or the group/population. All are relevant to CCTs. Modern clinical caries management can be seen as comprised of seven discrete but linked steps (Steps 2, 3, and 4 are directly concerned with measurement.): (1) 'Caries detection' represents a yes/no decision as to whether caries is present; (2) lesion measurement assesses defined stages of the caries process, taking into account the histopatholgical morphology and appearance of different sizes and types of lesion and the diagnostic threshold(s) being used; (3) lesion monitoring by repeated measures at a series of examinations is used when lesions are less advanced than the stage judged to require operative intervention (A comparison of serial measurements permits the efficacy of preventive care aiming either to arrest or to reverse the lesion to be assessed.); (4) caries activity measures would be very valuable, but are relatively poorly developed and tested at present; (5) diagnosis, prognosis, and clinical decision-making are the important human processes in which all the information obtained from steps 1 to 4 is synthesised; (6) interventions/treatments, both preventive and operative, are now routinely used for caries management; and (7) outcome of caries control/management assesses caries management by examining evidence on the long-term outcomes. A challenge for the future is to define a range of optimal caries measurement methods--in use or in development in recent trials, in clinical practice, and/or in caries epidemiology--that will best contribute to more efficient, modern caries clinical trials.