A great diversity of gastro-intestinal nematodes parasitise man and his animals. Each nematode is antigenically highly complex with distinct developmental stages. The parasites occupy a range of niches in the host gut and have a variety of feeding habits. Hosts have evolved only partially successful local immune responses. The host and parasite factors modulating these responses are described. Strategies for vaccination are surveyed. It is argued that the best route to successful vaccines will be to seek parasite immunogens which are not normally seen by the host's immune system in the course of infection. Routes to be followed in identifying and characterising such immunogens are proposed and methods for evaluating and maximising the immune response are surveyed. The success of this strategy is illustrated by reference to an experimental molecular vaccine against Haemonchus contortus. Strategies for the use of vaccines and methods for their production in vitro are reviewed.