Genetically engineered crops have become a visible part of the US agricultural landscape. The first transgenic varieties in or near commercial production have been modified for a range of characteristics conferring improved agronomic performance, herbicide tolerance, pest and disease resistance, handling and storage properties, as well as other traits. However the use of biotechnology to address constraints in agricultural production brings with it questions regarding the potential of genetically modified organisms (GMOs) to cause unacceptable impacts on the environment.Among the ecological issues associated with transgenic crops is the possibility that some newly introduced traits, such as pest or pathogen resistance, could confer added fitness to the crop. As a result, the crop may gain weedy characteristics if its ability to survive and spread outside of cultivation is enhanced. A second issue arises if such crops are grown in the vicinity of compatible wild or weedy related species; transfer of the trait by natural hybridization may produce hybrid progeny that are more aggressive or more difficult to control. These issues are no longer hypothetical, as at least seven groups of crops being engineered for pest resistance are known to have sexually compatible wild or weedy relatives in the US.Pest resistance has been a primary objective of farmers and breeders throughout the history of agriculture. Genes identified in wild germplasm or recovered as spontaneous or induced mutations have been incorporated into cultivated varieties of many major crop species. This process is now being supplemented by the techniques of genetic engineering, and dozens of crop species are being engineered for improved pest resistance. Pre-release risk assessment of these crops addresses the question: Does releasing such crops pose any special risk of creating or exacerbating a weed problem?Assessing the potential for transgenic pest resistant crops to become problem weeds, or to enhance the weediness of nearby sexually compatible relatives, is a complex task. Information is required from many disciplines - weed science, agronomy, population biology and genetics, entomology, plant breeding, ecology, plant pathology, molecular biology, and more. Scientific evidence in support of informed risk assessment and decision making thus lies in the collective knowledge of experts from these fields.The workshop on Ecological Effects of Pest Resistance Genes in Managed Ecosystems was organized to promote multidisciplinary discussions that would lead to a synthesis of what we already know, and what we don't know, regarding the environmental impact of pest resistant crops. In so doing, the workshop provided an opportunity to reexamine a key issue related to the responsible development and use of agricultural biotechnology products.