During the last ten years the principal biological factors which have been used as indicators of future caries activity are counts of mutans streptococci and lactobacilli. The evidence that these micro-organisms play a key role in the development of dental caries has now become very strong. Methods for the identification and enumeration of mutans streptococci and lactobacilli in saliva and plaque material have been developed which are both feasible and reliable. In a few studies counts of these micro-organisms together with other factors have been related to caries incidence. The validity of the various tests show a large variation but in some studies the predictive value has been high. The best prediction of future caries activity is obtained by the combination of factors which are important contributors to the pathogenesis of dental caries.