Surprising though it seems, the world faces almost as great a threat today from arthropod-borne diseases as it did in the heady days of the 1950s when global eradication of such diseases by eliminating their vectors with synthetic insecticides, particularly DDT, seemed a real possibility. Malaria, for example, still causes tremendous morbidity and mortality throughout the world, especially in Africa. Knowledge of the biology of insect and arachnid disease vectors is arguably more important now than it has ever been. Biological research directed at the development of better methods of control becomes even more important in the light of the partial failure of many control schemes that are based on insecticide- although not all is gloom, since basic biological studies have contributed enormously to the outstanding success of international control programmes such as the vast Onchocerciasis Control Programme in West Africa. It is a sine qua non for proper understanding of the epidemiology and successful vector control of any human disease transmitted by an arthropod that all concerned with the problem - medical entomologist, parasitologist, field technician - have a good basic understanding of the arthropod's biology. Knowledge will be needed not only of its direct relationship to any parasite or pathogen that it transmits but also of its structure, its life history and its behaviour - in short, its natural history. Above all, it will be necessary to be sure that it is correctly identified.