In recent years, dosage forms with sustained release have obtained a significant importance. The technological possibilities for manufacturing are described as coating, embedding and matrix procedures. The range of auxiliary substances, which are responsible for the retardation of drug activity, reaches from lipophilic compounds as lipoids, fatty alcohols and compounds which are forming hydrogels as cellulose derivatives and natural polysaccharides to synthetic polymers derived from acrylic acid. The formulation of the dosage forms requires particular care in respect to the amount of initial and maintenance doses. On account of technological processes, for example during manufacturing of tablets, under certain circumstances the liberation rate is altered. In vitro test methods allow comparisons only then when the results can be counter-checked by in vitro experiments. The release of drug follows different mechanisms, which are described, entirely or in part, to be reactions following the time law of zero or first order. In special cases, a linear correlation is observed as a function of square root of time. The calculation of given special equations for events within the dosage form is feasible from blood-level values.