Addressing the importance of quantum mechanical tunneling in enzyme-catalyzed reactions, this review covers the tunneling and recrossing effects in enzymatic reactions as well as recent theoretical developments. First, previous reviews covering tunneling and recrossing in enzymes are outlined. Second, a historical overview of the existing experimental evidence suggesting the importance of quantum tunneling in enzymatic reactions is given. Third, conceptual models proposed and widely used to interpret these experimental kinetics data in terms of tunneling are discussed. Fourth, sophisticated quantitative models capable of revealing detailed tunneling mechanisms at the atomic level are explained and compared. Finally, a survey of important enzyme systems that have been studied with the most complete theories is presented.