In the Netherlands, criteria have been set for field trials with genetically modified (GM) plants. These criteria are based on the step-by-step principle as described in European Directive 2001/18/EC. Three categories of field trials are defined. The first category concerns small-scale field trials with GM plants that are not well characterised on the molecular and phenotypic level. Confinement measures are applied in order to limit potential adverse effects to the field location. The second category consists of small-scale field experiments with GM plants that are better characterized. Confinement measures are no longer necessary since sufficient information is available to assess potential adverse effects on human health and the environment for these trials. The third category consists of large-scale field trials with fully characterised GM plants and without the need of confinement measures. For each category, a new permit is required. This system has been used for several crops, including apple and poplar trees and may also be applicable for trials with GM forest trees.