Since the mid-1940s, herbicides have been the most cost-effective and efficient method of weed control for agronomic crops. Today, herbicide-resistant weeds, in combination with a decline in industry discovery programmes and a cessation in discovery of new herbicide sites of action, threaten the continued utility of herbicides. Weeds have evolved resistance to 160 different herbicide-active ingredients (23 of the 26 known herbicide sites of action) in 86 crops and in 66 countries. This chapter reviews the various kinds of herbicide resistance, and then considers resistant weeds by site of action, crop, region and weed family. It considers the available strategies for managing herbicide-resistant weeds, but concludes that although herbicides are likely to remain the backbone of agronomic weed control for the next 30 years, their utility will steadily decline, and we need to begin working on new weed control technologies that will eventually replace herbicides.