Field trials are an important step in the experimental research with and commercial development of genetically modified (GM) plants, including GM trees. Field trials with GM plants in the European Union (EU) are subject to authorisation granted on the basis of an environmental risk assessment (ERA). Data requirement for the ERA varies depending on the purpose of the field trial and the level of knowledge on the GM plant and its environmental impact. In the Netherlands a step-by-step approach has been developed for the categorisation of field trials. Under this approach the confinement of GM plants in a field trial can be gradually decreased and the scale of the trial increased in a step-wise manner at the same time that knowledge on the GM plant and its environmental interactions increases. Very few other countries seem to apply a similar classification of field trials. We argue that a formal step-by-step approach may be a helpful tool to facilitate the approval process for field trials of GM plants and the collection of relevant ata/material for the ERA without compromising the environmental safety, and that this approach is also applicable to field trials with GM trees.