The effect of genetically modified (GM) plants on arthropod natural enemies of pest insects is discussed. The paper primarily concentrates on the potential impact of the insect resistant crops currently grown commercially in several countries, which express delta -endotoxin genes derived from Bacillus thuringiensis. The following factors influencing the impact of GM crops on the population dynamics of natural enemies are considered: the level of plant resistance; promoters and transgene expression; specificity of the transgene product; the presence of susceptible plants; and the plants species, geographical location and crop management. It is concluded that a thorough risk-assessment of every GM plant to be released is essential and, in addition, post-approval monitoring of natural enemies should be employed once insect resistant GM plants are grown on a large commercial scale.