Forest trees are fundamental components of our environment, mainly due to their long lifetime and important role in forest ecology. In the past, some non-native tree species and taxa from traditional breeding have induced severe environmental impacts such as biological invasion, changes in the 'gene pool', and spread of diseases in forestry. Genetically modified trees obtained in different research groups worldwide are particularly confronted with increased concerns regarding biosafety issues. In the light of current biosafety research worldwide, various threats facing forests and natural tree populations are evaluated in this review: biological invasions, horizontal gene transfer, vertical gene transfer and effects on other organisms. Results available from groups working in biosafety research and risk avoidance using forest trees, with emphasis on transgenic trees, are reviewed. Independent biosafety research as well as the establishment of biosafety research programs for forest trees financed by national and international authorities is now more important than ever before. Biosafety problems detected in the past clearly show the importance of a prior case-by-case evaluation of non-native species, new taxa and also genetically modified trees according to the precautionary principle before their release to avoid risks to the environment and human health.