Based on the precautionary principle, legislation addressing environmental aspects of researchon, development and marketing of genetically modified organisms (GMOs) was established in the early nineties. At that moment, limited knowledge on environmental effects of GMOs was available. Since then a vast amount of data has been generated. Both applicants and research institutes in Europe and elsewhere studied a diversity of aspects relating to environmental safety. In Europe and in the Netherlands in particular many field trials and clinical trials have been conducted. Much experience is gained on genetically modified (GM) crops that have been commercialized in several parts of the world. However, some topics may still lack sufficient high quality data. The National Institute for Public Health and the Environment (RIVMRijksinstituut voor Volksgezondheid en Milieu) commissioned PERSEUS bvba to inventory the areas of concern in environmental risk assessments (ERA) for development and market introduction, to evaluate the type of information necessary to perform ERAs both in the field of GM plants and gene therapy for humans, to identify areas in which our understanding has evolved to a level that provides confidence for conclusions in the ERA, to point out remaining areas of uncertainty and to recommend topics for further studies and initiatives.