The removal of leaf area will reduce the photosynthetic activity of any crop.  Economic yield loss resulting from a defoliation event will depend on the ability of the crop to compensate for the reduced leaf area.  Compensation may occur via increased photosynthetic activity of remaining leaves, additional leaf production after the defoliation event, and/or surplus leaf area prior to the defoliation event.  Factors causing defoliation include, but are not limited to, drought, hail, insects, disease, and herbivores.  The timing of these events and the crop stage that they occur will largely affect whether or not the crop will recover or suffer economic loss.  New threats such as soybean rust are causing scientists, crop advisors, and other practitioners to reassess past research and recommendations.  Past and ongoing research and an attempt to quantify the crop response to defoliation at a range of crop stages are reviewed.  Physiological implications of crop defoliation, relationships of leaf area index/light interception to defoliation events, interaction of defoliation events with crop stage, yield losses due to insect defoliation and soybean rust, weed management implications, seed and grain quality response, and risk management are addressed.