Several organizations around the world are using recombinant DNA technology to genctically engineer tree species to improve or customize important silvicultural traits such as pest tolerance, wood characteristics, flower sterility, and growth rates. Already, field testings of transgenic trees have occurred in the United States, France and the United Kingdom. In Canada, it is expected that field applications for field testing will be submitted within a year. For the scientific assessment of risks for this type of plants, several biological differences impact on the results of such an excrcise. Trees are long lived perennial plants with in several cases very particular flower biology. Furthermore, most species are semi-domesticated and are used close to natural reservoirs of genetic diversity. Because of these reasons, risk assessment is more complex than with crop plants and issues of containment become more important. However, forestry in the world has a long history of importation of foreign genomes that did not result in adverse effects to the environment. Furthermore, transgenic trees are more likely to be used in artificial ecosystems (i.e., intensively managed plantations) because of the investment required to produce them. Particular attention will have to be given to traits with potential risks to the environment such as frost and drought tolerance that could extend the natural habitat of the transgenic trees or pest tolerance that could have an effect on the food chain. The Canadian Forest Service has initiated a research program to evaluate the potential for risks with the release of transgenic trees. Transgenic trees of poplar, black spruce and tamarack containing marker genes are now available and are being multiplied to establish field tests and to develop microcosms. Also, transgenic tissue culture lines of conifers are being produced that contain synthetic Bacillus thuringiensis -endotoxin gene and antifungal peptide genes for use in ecological impact research. With the increasing demand on wood and wood products, it is critical to improve the productivity of commercial forests while protecting untouched natural forests. Transgenic trees are part of the solution but proper risk assessment must be performed to avoid any negative impact.