Abstract The dynamic nature of caries lesion progression may require that classification of caries lesions makes distinctions in activity status. The aim of the present review was to compile and discuss the literature which pertains to clinical assessment of caries lesion activity, and to examine whether recent developments in microbiological research may justify the use of microbiological methods for evaluation of caries activity. Clinical observations suggest that caries lesion progression can be arrested at any stage of lesion development, provided that clinically plaque-free conditions are obtained. However, there is no universal level of oral hygiene to be recommended. The diagnoses “active” and “inactive/arrested” caries have been validated by a range of histological and chemical methods which have supported a separation into distinct clinical categories. Simple microbiological methods have so far not been useful in differentiating between active and inactive caries lesions. Very few studies have evaluated the inter- and intra-examiner reliability of caries diagnostic criteria based on assessment of the activity state of lesions, but recent data indicate that active and inactive caries lesions can be diagnosed with a high degree of reliability. A decision-making tree for dental caries is presented by means of which it is possible to associate the assessment of caries lesion activity with an appropriate treatment modality. It is concluded that research into better methods of assessing caries lesion activity clinically should be stimulated.