Crops or desired plant species co-occur with undesired species, and the co-occurring species thereby come to be classed as weeds. This human-imposed classification is based on the perception that there is an interaction that results in some negative effect of the weed on the crop or desired species. This chapter offers an evolutionary perspective on crop–weed interactions and examines the nature of shared resource pools between desired crops and weeds. The chapter addresses the effects of direct competition between weeds and crops for resources, the indirect effects of competition and the spatial and temporal dynamics of crop–weed interaction.