In order to bring definitions of drug dosage forms up to date, it was necessary for the French Pharmacopoeia to propose assays allowing the quality control of the dosage forms to be based on the kinetics of drug release in vitro. Currently, five examples can be cited of dosage forms that can be characterized by release in vitro. (1) Oral solid dosage forms--for tablets, all the parameters of powders before compression (e.g., flowability, tableting properties) are being studied in addition to the dissolution tests, (2) Rectal dosage forms--the disintegration test of suppositories will be discarded and a new dissolution test using a special flow-through cell is now being studied. (3) Inhalations--since particle diameter is the most important factor for inhalation activity, a method has been developed to give the correct answer to this question. (4) Modified release drug dosage forms--these have been defined separately from the conventional forms. For the peroral route, they are: (i) accelerated release drug dosage forms, (ii) sustained release drug dosage forms and (iii) delayed release drug dosage forms. To emphasize the differences in the release kinetics, use of the paddle method, well known in the USP, and the flow-through cell has been suggested and described in the European Pharmacopoeia. Some associations and/or in vitro-in vivo correlations have increased the interest in the last method. (5) Transdermal delivery systems--these are defined separately from plaster and sticking-plaster. The use of a cell method was suggested to study the drug release and some comparisons between different techniques are presented.