The relationship of nematodes and nematomorphs with grasshoppers and locusts is reviewed, emphasizing the actual or potential role of these parasitoids in microbial management. There are records of mermithids parasitizing grasshoppers worldwide, and they are considered important biological control agents in some grassland ecosystems of Europe, North and South America, Papua New Guinea, New Zealand, and Australia. Nematomorphs, although widely distributed, are uncommon parasitoids of grasshoppers, and their dependence on free-standing water for host infection and apparent host specificity are considered drawbacks to their use in biological control programmes. The ascaridids, spirarids, and acanthocephalans are parasites of birds and mammals, and may use grasshoppers as intermediate hosts. They have been shown to debilitate grasshopper hosts in laboratory studies, but their primary role as parasites of vertebrates precludes any consideration as biological control agents. Rhabditids do not naturally parasitize grasshoppers, but recent advances in mass-culturing techniques have given them a potential role as bioinsecticides for the control of grasshoppers. Quantitative data on the effects of nematodes and nematomorphs on agricultural pests, including grasshoppers and locusts, are generally lacking. However, there is evidence that some, particularly mermithids, are important in the population dynamics of grasshoppers and locusts. Keys to the identification of the various 'worms' found in grasshoppers and locusts are provided, including keys to the species of mermithids.