The general properties of flavins in free solution and when bound to flavoproteins as cofactors are analysed. The extremely high chemical versatility of flavins is reflected in the remarkable versatility of flavoproteins, when considered as a whole. However, each flavoprotein is also characterized by a strict specificity, thus implying that one of the most important roles of the protein component is to limit the wide range of possible flavin–protein interactions to those beneficial to the reaction to be catalysed.Many attempts have been made to achieve a rational classification of flavoproteins, depending on their different properties. However, when general classifications are based on the different possible catalytic mechanisms, in most cases there is uncertainty, owing to the wide range of reaction mechanisms potentially available to the flavin cofactor. Despite the significant accumulation of information in recent years on catalysis, biomimetics and structural studies, the factors that determine the specificity of flavoproteins are still poorly understood.