The Swiss Expert Committee for Biosafety SECB advises on issues related to the protection of people and the environment in the areas of biotechnology and gene technology.  Consequently, the SECB counsels the authorities on permit applications for field trials and commercial cultivation of genetically modified (GM) crops and issues statements on safety aspects of GM crops.The SECB has appointed the Agroscope Reckenholz-Tänikon Research Station ART to review the available scientific data on ecological effects of commercially grown GM crops to enable the Committee to apply the results obtained from the worldwide experience with GM crops as a basis for its decision-making process. The present study focuses on insect-resistant maize and herbicide-tolerant soybean and oilseed rape, respectively, representing three of the major GM crops of significance for Swiss agriculture, and their possible impact on the environment. Peer-reviewed journals, scientific books and reports from international organizations have been the main source of data for the study.We believe that the study offers a science-based and broad overview of the worldwide situation of possible impacts of GM crops on the environment. We regret, however, that most of the data originate from large-scale field trials rather than from commercial cultivation. This lack of data from commercial cultivation is mostly due to the fact that legal requirements for a post-market monitoring of GM crops are missing in the major GM crop-producing countries. Furthermore, the conventional farming practices in those countries differ largely from the integrated-production system mainly applied in Switzerland, which makes the transferability of data difficult, especially when internationally recognized baselines for the comparison of environmental effects of GM crops in the context of modern agricultural systems are not available.Some committee members would have preferred a stronger focus on remaining uncertainties and knowledge gaps, as well as on the controversial discussions among scientists concerning the interpretation of scientific data. Therefore, the study does not represent the position of all committee members. A minority of the committee members dissents with the conclusion that the data available up to now do not provide any scientific evidence for harm caused to the environment by commercial cultivation of GM crops.Nevertheless, we believe that the Committee will profit from the worldwide experience drawn from the cultivation of GM crops reviewed in this study when assessing the risk of field trials with GM crops, taking into account the precautionary approach and a sustainable agricultural development in Switzerland. We hope that this study contributes to a sound discussion on benefits and risks of GM crops.