The introduction of Roundup Ready@ cotton and the future release of other herbicide tolerant cotton varieties will allow more flexible weed management with greater emphasis on postemergence herbicides for weed control. To optimize the use of these herbicides and to minimize the risks associated with herbicide resistance and species shift, these technologies need to be introduced as part of an integrated weed management(IWM) program. The critical period for weed control is an important component of IWM and provides growers with information on when weeds are likely to interfere with crop growth, and when weed management strategies should be targeted to avoid yield reductions. Mimic weeds are plants that have similar biomass and leaf area to actual weeds encountered in the field and allow the development of critical periods for weed control that are applicable to a range of species, based on their competitive impact on cotton. The critical period for weed control allows for more flexible and effective management packages to be developed for a range of weeds and is similar conceptualIy to that already established for insect control using IPM in the Australian cotton industry.