For the last 20 years, as a complement to biochemical approaches, lignin-modified transgenic trees have proven to be invaluable tools for deciphering the lignin biosynthetic pathway and increasing our knowledge about the role of lignins in wood-specific properties important for tree physiology. In addition, their evaluation in the field has been very useful for the identification of possible routes to improve wood properties for specific industrial uses, initially pulp and paper making and now biofuel production. In this chapter, we review our present knowledge gained through numerous studies analysing transgenic trees, with emphasis on field trial evaluations, for assessing the effects of lignin modification on wood properties and tree physiology. All the data presented here led to the conclusion that, prior to use in forestry for pulp and paper or bioenergy production, the fitness of lignin-modified transgenic trees required careful evaluation in a variety of field environments. Although limited in number, environmental studies failed to indicate any strong effects of lignin-modified transgenic trees on the ecosystem. Finally, this review helps to identify some promising options where biotech trees could be used to further optimize the production of wood for human uses.