In recent years, researchers investigating innate immunity have begun to use C. elegans as a new model system. The worm has been found to mount protective responses to a variety of fungal and bacterial pathogens. Four signalling pathways involved in such responses have been identified so far: the p38 MAP kinase pathway, the programmed cell death pathway, the TGF-beta pathway and the DAF-2 insulin/IGF-I like signalling pathway. Activation of these pathways can lead to the production of immune effector molecules such as lysozymes, lipases and saposin-like proteins, which can act directly against the invading microorganisms. The signalling pathways used and the effectors produced depend on the nature of the infection, indicating that the worm can detect and discriminate between infecting microorganisms. However, the molecules involved in recognition of pathogens have yet to be identified. The worm genome encodes various proteins which might have this recognition function, such as numerous proteins containing C-type lectin domains. These and other candidates are discussed.