Drugs are able to activate the immune system, which may generate hypersensitivity states in individuals. This article first deals with the critical processes that are involved in drug sensitizations: what are the specific features of drugs as immunogens; how are drugs recognized as non-self by activated immune cells; what is the spontaneous outcome of an immune response to drugs in individuals; does a genetic predisposition regulate the immune response to drug antigens; how much are biotransformation processes involved in the immunogenicity of drugs? The second part is mostly devoted to the biological investigation of drug sensitizations: what are we looking for and why; are all the available methods equally suitable for routine diagnosis; what are the major methological problems that we have to deal with in investigating patients who have presented with symptoms which are clinically suspected to be of immuno-allergic origin?