With the widespread adoption of herbicide-resistant (HR) crops, research is increasing required to evaluate their impact on agroecosystems. These crops will solve some weed problems for farmers, but will create new ones as well. Potential benefits and risks of HR crops are described. HR crops have the potential to change weed communities via escape and proliferation as ‘weedy’ volunteers, hybridization with and resistance gene infiltration into related weedy relatives, and increased selection for evolution of resistance due to frequent herbicide group use. Controlling HR crop volunteers in the crop rotation will require careful planning, particularly those plants with multiple resistance due to pollen flow. Sound agronomic practices and good record keeping is required by farmers to effectively manage these new potential weed problems.