The environmental risks of many transgenic crops can be evaluated without additional experimentation by using already available information on the biology of the crop, the presence of compatible relatives, and the transgene phenotype. The level of crop invasiveness and the location of compatible relatives can be determined by consulting local floras and the crop literature. Decisions about invasiveness can be bolstered by determining the number of weediness traits carried by the crop and its congeners. The potential impact of transgenes can be ranked by their likely effect on reproductive success, ranging from neutral to advantageous to detrimental. This scheme can identify not only the low-risk transgene–crop combinations that are safe to deploy but also those that either are too dangerous to release or require additional experimentation.