Competition from weeds is the most important of all biological factors that reduce agricultural crop yield. This occurs primarily because weeds use resources that would otherwise be available to the crop. The magnitude of yield loss is affected by numerous agronomic and environmental factors, most importantly, weed density and time of emergence relative to the crop. Practices that (1) reduce the density of weeds, (2) maximise occupation of space or uptake of resources by the crop or (3) establish an early-season size advantage of the crop over the weeds will minimise the competitive effects of weeds on crops. Longer term management of crop–weed competition can be achieved through crop rotations, specifically crop sequences that reduce the weed seed bank, and therefore seedling density, and prevent proliferation of perennial weeds. Key ConceptsKey Concepts * Plant growth requires sunlight, water and nutrients, which in turn are converted into biomass that captures additional resources. * In crop or weed monocultures, increasing density increases total production to a maximum that is determined by the resource status of the site, generally with a corresponding decrease in per plant mass. * In crop–weed mixtures, increasing weed density results in large initial reductions in crop yield, again reaching an asymptote of maximum yield loss that varies among crop species, environments and weed cohorts. * Farmers manage crop–weed competition by (1) reducing weed density (i.e. ‘weeding’), (2) establishing an early-season competitive advantage to the crop and (3) maximising resource capture by the crop using competitive species, competitive cultivars, high sowing densities, optimal spatial arrangement, intercropping complimentary species or transplanting.