The weeds of farmland are a diverse and economically valuable group of plants that have formed a durable botanical thread since before the land was first opened for cereals and pasture. Weeds are an economic burden when they take resources from crops, interfere with harvesting or contaminate yield. But only a few plant species cause economic loss at any time, and those few change over the years in response to trends in cropping. Weeds are not all burden: many species provide food and shelter for insects, some beneficial to agriculture. They mop up fertilisers missed by the crop, so reduce pollution. Leguminous weeds can fix nitrogen from the air, so reduce the need for mineral fertiliser. Many weed species have nutritional or pharmaceutical properties that are still valued in cultures throughout the world. Fifty years of chemical weedkillers have not exterminated weeds in north west Europe. The beneficials have been reduced, but the aggressive few have simply adapted. The future lies in limiting the economic burden of weeds, encouraging their beneficial aspects and discovering new economic products in their biochemistry and structure.