Currently a range of systems and classifications are used by clinicians, epidemiologists and clinical research workers to sub-divide carious lesions into different grades. These systems are based on the depth of the lesion and/or the presence/absence of macroscopic cavitation. In order to improve upon the meaningfulness and comparability of such systems in the light of increasing knowledge about the disease process, lesion behaviour and caries management options, the authors propose a new system of categorisation that differentiates between lesions which normally require operative intervention and those which do not. In future, it is proposed that in addition to existing conventional criteria, diagnostic systems should also allow results to be expressed in terms of 1) lesions for which appropriate non-invasive Preventive Care Is Advised (PCA lesions) and 2) lesions for which Operative Care Is Advised (OCA lesions). Locally acceptable sub-divisions may have to be developed and agreed for certain specific applications. The adoption of this additional nomenclature and approach in the future should aid communications between the different groups involved in caries diagnosis and research. It may facilitate the dissemination of research findings, as well as helping to "drive" researchers working on caries diagnostic methods to focus on techniques which will aid in the accurate assessment of lesion activity and behaviour. Such a focus should also increase our understanding of treatment decision making and promote the development of clinical guidelines. The facility to retain existing criteria in parallel, for local and other purposes, would preserve comparability with data collected previously.