AbstractThe essential requirements for enzyme assays are described and frequently occurring errors and pitfalls as well as their avoidance are discussed. The main factors, which must be considered for assaying enzymes, are temperature, pH, ionic strength and the proper concentrations of the essential components like substrates and enzymes. Standardization of these parameters would be desirable, but the diversity of the features of different enzymes prevents unification of assay conditions. Nevertheless, many enzymes, especially those from mammalian sources, possess a pH optimum near the physiological pH of 7.5, and the body temperature of about 37°C can serve as assay temperature, although because of experimental reasons frequently 25°C is preferred. But in many cases the particular features of the individual enzyme dictate special assay conditions, which can deviate considerably from recommended conditions.In addition, exact values for the concentrations of assay components such as substrates and enzymes cannot be given, unless general rules depending on the relative degree of saturation can be stated. Rules for performing the enzyme assay, appropriate handling, methodical aspects, preparation of assay mixtures and blanks, choice of the assay time, are discussed and suggestions to avoid frequent and trivial errors are given. Particularities of more complex enzyme assays, including reversible reactions and coupled tests are considered.Finally the treatment of experimental data to estimate the enzyme activity is described. The procedure for determining the initial enzyme velocity and its transformation into defined enzyme units as well as suggestions for documentation of the results are presented.