Genetically modified (GM) maize and cotton varieties that express insecticidal proteins derived from Bacillus thuringiensis (Bt) have become an important component in integrated pest management programmes worldwide. A number of other crops producing Bt toxins, or more broad-spectrum insecticidal proteins, are likely to enter commercial production in the near future. Because insecticidal GM crops target insect pests, an important part of the environmental risk assessment is their potential impact on nontarget arthropods. Those include protected species and organisms providing important ecological services such as biological control of herbivores. Non-target arthropods can be exposed to the plant-produced insecticidal proteins through various routes, but mainly by feeding on GM plant material or herbivores that have consumed GM plant material. The Bt proteins produced in today's GM plants appear to have no direct effects on natural enemies due to their narrow spectrum of activity. Furthermore, it has become clear that in crop systems where the deployment of Bt varieties has led to a decline in insecticide use, biological control organisms have benefited significantly. Future GM plants that produce broader-spectrum insecticidal proteins will need to be assessed for their potential non-target effects case by case and compared to the impact of the conventional pest control methods that they replace.