A continuous development in plant biotechnology including gene technology has been observable during the past 20 years. Different methods elaborated with model plants were also applied to forest trees on a larger scale. Whereas in the beginning the meaning of the term “plant biotechnology” embraced a wide variety of meanings like, e. g. , regeneration of plantlets via tissue culture, embryo rescue, somatic embryogenesis and gene transfer, the focus of this term has changed more and more. Nowadays, it is the transfer of genes which comes into mind when plant biotechnology is discussed, including of course the evaluation of all challenges and risks related to gene transfer methods. Compared with annual plants, especially in the field of agriculture, the work and the progress with transgenic trees is still in its infancy. Nevertheless, but often unnoticed by the scientific community, there are a few countries which already allow the commercial use of a restricted number of transgenic tree clones after different critical steps of approval. This and the ongoing improvement in transgenic research in trees led to the idea of preparing a summary of the present state of the art from different points of view. With the help of a number of authors directly or indirectly involved in tree transgenesis, this book was produced.