Risk is defined as the chance of a specific event x the effect of the event. Large-scale cultivation of transgenic herbicide resistant crops can result in poorer weed control of less susceptible weeds. Another risk is reduced attention (and money) for research on reducing the environmental impact of weed control methods. Large-scale cultivation of transgenic herbicide resistant crops can also result in a delaying of weed control (thus allowing the development of harmful plant pathogens) and a reduction of biodiversity in field borders (hiding place of natural enemies). Knowledge of interactions between different types of modification, genotypes, environmental factors and the susceptibility of the environment is required on a case by case base. The introduction of transgenic herbicide resistant crops in the Netherlands should be accompanied by a strict monitoring programme of laboratory, greenhouse and field studies on experimental farms.