Trees differ in a number of important characteristics from field crops, and these characteristics are also relevant for any risk assessment of genetically engineered (GE) trees. A review of the scientific literature shows that due to the complexity of trees as organisms with large habitats and numerous interactions, currently no meaningful and sufficient risk assessment of GE trees is possible, and that especially a trait-specific risk assessment is not appropriate. Both scientific literature and in-field experience show that contamination by and dispersal of GE trees will take place. Transgenic sterility is not an option to avoid the potential impacts posed by GE trees and their spread. Regulation of trees on a national level will not be sufficient because due to the large-scale dispersion of reproductive plant material, GE trees are likely to cross national borders. All this makes GE trees a compelling case for the application of the precautionary principle.