OBJECTIVE To describe the 'Caries Assessment Spectrum and Treatment' (CAST) instrument, its origin and content, and to provide an example of how results can be reported. METHODS The CAST instrument covers the complete range of stages of carious lesion progression: from no carious lesion, through caries protection (sealant) and lesion care (restoration) to lesions in enamel and dentine, the advanced stages of carious lesion progression in pulpal and its consequences on tooth-surrounding tissue and tooth loss due to caries. Using the RAND modified e-Delphi consensus method two independent panels, comprising a total of 56 epidemiologists from 27 countries, determined the face and content validity of CAST. Panellists assessed 17 statements related to the content, description, suitability and international use of CAST. Agreement of 75% or higher was required for reaching consensus on a statement. Construct validity and reliability testing of CAST have been carried out, but results are not yet available. RESULTS All 17 statements were approved by the panellists, who found the RAND modified e-Delphi consensus method suitable for achieving consensus. The CAST codes were ordered hierarchically. External validity was obtained. Reporting using CAST can be performed for orally healthy dentitions and those having morbidity and mortality. A DMF score can easily be calculated from the CAST codes, thereby enabling retention of the use of existing DMF scores. CONCLUSIONS The CAST instrument for use in epidemiological surveys is very promising. Face and content validation is obtained. Construct validity and reliability testing will be completed soon. A structure for reporting results in a comprehensive, pragmatic and easily understood way is being developed.