Following attack on an insect vector-perhaps with insecticide or by removing breeding sites-we need to know what reduction in disease can be expected. In general, reliable predictions of the epidemiological consequences of any entomological change are highly desirable. Such predictions require a measure of the capacity of an insect population to transmit disease. A popular approach is to try to quantify all the entomological components of transmission, in isolation from the parasitological components, and then calculate the maximum daily reproductive rate of the disease-known as the Vectorial Capacity. But to measure accurately all the entomological components is technically demanding. In this article, Chris Dye argues that epidemiological predictions could be made by including only the dominant entomological variables in an index-a streamlined Vertical Capacity-which can then be fine-tuned by closely matching entomological and parasitological observations.