The accompanying table summarizes the current status of resistance to insecticides by insect pests of public health importance in various regions of the world. There has been experimental confirmation of resistance in a number of species, and field observations of resistance are becoming increasingly common. No one who is now concerned with the control of insect pests of public health importance can fail to see the potential impact of resistance upon control programs. For example, a breakdown of the effectiveness of insecticides in controlling malaria mosquitoes might be quite disastrous in some areas of the world where the loss of acquired immunity during a period of satisfactory control has made the population increasingly susceptible to a break in the threshold of transmission. It is hoped that an intensified program of fundamental research on all aspects of this problem may provide a satisfactory solution before any such situation develops.