Many factors can interact with an insect infestation to cause damage and loss to alfalfa grown for forage. In the case of the potato leafhopper, Empoasca fabae (Harris), damage may be compounded by other stress factors such as lack of soil moisture, inadequate plant nutrition, timing and frequency of cutting, cultivar grown, plant disease, and other insect pests such as the alfalfa weevil, Hypera postica Gyllenhal, or meadow spittlebug, Philaenus spumarius (L.). The authors discuss these factors through a historical review of nearly 40 years of research at Purdue University on the potato leafhopper. Methods used to estimate damage and loss in field studies are shown and results are tabulated. They show comparisons of tolerance and high susceptibility to damage and loss with early cultivars, ‘Ranger’ and ‘Buffalo.’ ‘Ranger’ is shown to have considerably more tolerance to the potato leafhopper than ‘Buffalo,’ particularly under conditions of drought. The influence of moisture stress on the extent of damage is explained. The relationship of populations of this pest and the results of experimentation on in vitro digestibility of dry matter, forage yield, losses in protein, and accumulation of nonstructural carbohydrates in alfalfa roots is discussed with a series of regressions. Compensation by the alfalfa plant that causes it to produce more when infested by subeconomic pest population levels is cited. We also cite the results of research that has shown the losses that occur to crop vigor and production of subsequent harvests of alfalfa because of an uncontrolled economic infestation on a previous cutting. The authors conclude that the development of improved cultivars has lessened the impact of the potato leafhopper on alfalfa, but that it is still a serious pest. Tremendous strides have been made in understanding the nature and extent of losses caused by this insect pest. However, the perennial ecosystem of alfalfa with its continuance over winter for several years, along with four harvests per season allows for many interacting factors to complicate not only the determination of outbreaks, but the prediction of economic loss.