Genetically modified (GM) varieties of crops, notably soybean, maize, rape (canola) and cotton, were first grown commercially in 1996. In 2010 they occupied 148 million ha in 29 countries, mostly in the Americas and Asia but with an obvious absence in Europe where their introduction has been controversial due to concerns about environmental impairment and adverse impacts on human health. This paper reviews the published literature on the agronomic and environmental impact of GM crops in the last 15 years. Overall, the impact of GM crops has largely been agronomically and environmentally positive in both developed and developing world contexts. The often claimed negative impacts of GM crops have yet to materialize on large scales in the field. Agronomically, there have been yield increases per unit area, mainly due to reduced losses as a result of improved pest (i.e. insect) and weed control; in the case of conventional crops grown near GM varieties with insect resistance there have been benefits due to the so-called ‘halo’ effect. Environmentally, the decrease in insecticide use has benefited non-target and beneficial organisms while surface and groundwater contamination is less significant; human-health problems related to pesticide use have also declined. Equally important is the reduced carbon footprint as energy inputs are reduced. Of particular note, however, is the recognition that the success or longevity of GM crops is reliant on the speed with which resistance develops in target weeds and insects. However, resistance to GM-based plant resistance is already being detected in some pest populations and this suggests that scientists and farmers cannot be complacent. Current GM approaches are relatively transitory as a means of combating pests, as are conventional pesticides, and good management will determine how long this strategy proves positive. However, GM is a comparatively new science and the possibilities are considerable.