Transgenic and non-transgenic herbicide resistant crops are increasingly being planted commercially in North America. The widespread use of herbicide resistant crops has the potential to increase or alter the use pattern of the herbicide to which a crop is resistant, which has implications for the further evolution of resistant weed species. Gene flow from the resistant crop to related weeds is another potential impact of adoption of this technology, although the relative importance of this phenomenon to increases in weed resistance is unknown. Weed management in agricultural systems is achieved through a combination of methods that include prevention, eradication, and control (reduction of weed densities to economically acceptable levels). Methods of weed control include biological, chemical, cultural, and mechanical techniques, as well as combinations of these approaches. In many parts of the world, however, chemical control with herbicides is the predominant method of weed management. Currently in the US and worldwide, herbicides outrank insecticides and fungicides in both volume used and total sales. As use of herbicides has increased, increased cases of selection for resistance in weeds have been documented. Since the first reported case of weed resistance in 1970, 258 weed species have evolved resistance to one or more of 18 herbicide classes. One of the most recent cases documented is weed resistance to glyphosate, the active ingredient in Roundup©, which has been found in the US and four other countries. To preserve the utility of herbicides in agriculture, active resistance management is essential, using methods such as herbicide rotation, mixing herbicides with different mechanisms of action, and employing combinations of chemical and nonchemical control techniques.