It is common agronomic practice to perform a formal evaluation of the behaviour of new varieties under natural field conditions. Accordingly, shortly after the optimization of genetic engineering techniques on trees, a number of field trials were set up to assess GM trees modified for different genes. Here, we review the work that has been done in this arena in Europe over the last 25 years, and summarize what we learned from these experiments. GM tree field trials remain the exception rather than the rule in Europe. Several trials have been destroyed by anti-GMO activists and it is becoming increasingly difficult to obtain authorization for a GM tree field trial. These increasing constraints on GM tree trials within Europe are both surprising and counter-productive as we learned a lot from the past 25 years of experiments and the results were promisingly positive: (1) Phenotypic effects resulting from transgene expression in GM trees grown in the field appears to be stable, albeit variable; (2) most field studies have validated earlier observations made under greenhouse conditions, although in some cases the modification of target traits was less obvious in fluctuating field environments, and in a few cases had severe growth and developmental penalties; (3) non-target effects were consistently within the range of natural variation. Overall, the European GM tree field trials failed to exemplify any significant tangible risks. Based on this evidence, it seems appropriate that Europe should now move forward beyond small confined trials to larger scale experiments better fitted to a broader context of evaluation and environmental assessment.