Abstract Turnover number (TON) and turnover frequency (TOF) are the two concepts that can be encountered in the research of catalysis. It is normally accepted that TON is the number of reactant molecules that can be transformed per active site and TOF is the ratio of TON and the time to accomplish these transformations. TON and TOF can be used to evaluate the stability and activity of catalyst. In this paper, the TON and TOF in the homogeneous catalysis, heterogeneous catalysis and enzyme catalysis have been discussed and the related metrics, such as S/C, the molar percent of catalyst, TTN(total turnover number) and STY(site time yield) are also explained.
