Most of us are educated to consider nematodes as harmful organisms, attacking plants and parasitizing vertebrates, including man. Basically, nematodes can be arranged into several groups depending on their nourishment. Nematodes can parasitize spiders, leeches, annelids, crustaceans, molluscs, and other groups of invertebrates, including insects. When this parasitism results in death of the host, these forms are potential biological control agents. When the hosts happen to be insect pests of man, then the possibility of control becomes still more intriguing. This book examines those nematodes which are good candidates for the biological control of insects, either alone or in conjunction with other pest management systems. It highlights three criteria that a nematode must meet. The book discusses each nematode according to the following outline, which may vary depending on the amount of information available: description, bionomics (life cycle and ecology), host range, culture, and application.