Various incentives will progressively increase the part of generic drugs on the Belgian market place and will more and more confront the prescriber with the problem of substitution. The equivalence between generic and brand name products and the fear, justified or not, at efficacy or patient safety levels in case of substitution are discussed on the basis of biopharmacy and pharmacokinetic concepts in connection with modalities generally adopted when requesting marketing authorisation. Considering the extent of their utilisation at international level and the low frequency of clinically significant problems linked to bioequivalency, limited to a small number of substances, prescribers should be soon released from their apprehensions on the quality of generic drugs. On another hand, they must be stimulated to be vigilant when they substitute a drug by any other one, especially in the cases of drugs, classes of drugs, or pharmaceutical forms considered as critical and when occurring to patients whose disease or physiological status are able to amplify bioequivalence problems.