"Biotherapies" include both biopharmaceuticals and cell and gene therapies. Biopharmaceuticals are macromolecules created by biotechnologies. In the case of monoclonal antibodies, the starting dose to be given to Humans is difficult to select because there may be no relevant animal model. At the time of registration, the knowledge of the mechanism of action of monoclonal antibodies is insufficient and cohort follow up studies are needed. Genetic predisposition and pharmacokinetics are interindividual sources of variability. Some of the adverse drug reactions are predictable but others are unexpected or even paradoxical. Cell and "ex vivo" gene therapies consist in the manipulation of collected cells and their infusion in autologous or allogenic clinical settings. The methodology of the clinical development of drugs cannot be readily applied to these therapies. On the other hand, "in vivo" gene therapy uses vectors or macromolecules which can be considered as biopharmaceuticals.