The extent to which transgene flow from plantations can be effectively predicted, managed, and monitored, will be a critical biological and social factor influencing the adoption of transgenic plantations. Studies of historical and contemporary gene flow levels, via genetic structure surveys and parentage analyses, demonstrate that gene flow is generally extensive in both wind- and animal-pollinated forest tree species. Organelle genome studies have shown that pollen is by far the greatest source of gene dispersal, particularly for tree species with limited seed movement.Marker studies, however, give little insight into the extent and consequences of gene flow in actual deployment scenarios. For example, despite the potential for extensive gene flow, there appears to be extremely low levels of 'genetic pollution' of wild poplar stands by plantations of hybrids. Factors such as dilution by regional seed and pollen clouds, deleterious impacts of genotype backgrounds, engineered sterility genes, and the effects of transgenes on competitiveness in the wild, can greatly influence observed levels. Case-by-case analyses are required for useful predictions.We introduce a spatial simulation model called STEVE that we have used to estimate the levels of future transgene flow from poplar plantations in the United States Pacific Northwest. It synthesizes data from a variety of ecological and genetic processes, and permits virtual experiments that investigate how diverse genetic, ecological, and management factors might influence the magnitude and variance of gene flow over a 50 to 100 year period. Similar approaches could be used elsewhere to help identify priority research needs, suggest means for mitigation where warranted, and to aid in design of monitoring programs for large-scale research and commercial applications.