The science of chemotherapy has been built upon the seminal concepts arising from pioneering studies, notably by Paul Ehrlich, carried out nearly 100 years ago. The investigations conducted in the early decades of this century were of major significance for antiparasite chemotherapy as they resulted in the discovery of a variety of drugs for treating human and animal diseases. Derivatives of many of these drugs are still being used today, which, it could be claimed, reflects the outstanding success of the early approaches. However, it also highlights the relatively poor record of drug discovery in more recent decades. While major advances were taking place in the development of drugs and vaccines for bacterial, fungal and viral diseases, the discovery of new drugs for parasitic infections progressed relatively slowly. It would be wrong, however, to give an impression that there were no advances. The discovery of the avermectins and praziquantel has had an enormous impact on anthelmintic therapy, and new classes of antiprotozoal drugs have been developed for both prophylaxis and treatment of infections. Nevertheless, major problems remain and for some parasitic diseases the situation now is worse than it was 20 years ago. Malaria is perhaps the best example in this respect. The advent of chloroquine resistance in Plasmodium falciparum has resulted in a major public health problem. In addition, the increase in importance of some diseases, notably those associated with immunosuppression in AIDS, has again highlighted the limitations of antiparasite chemotherapy. New drugs are needed urgently for a number of parasitic diseases.