Parasitic protozoa and helminths are a diverse group of organisms which together form a major cause of infectious disease in humans and livestock. Studies in animal models have revealed that T lymphocytes and the cytokines they produce play a crucial role in determining the outcome of parasitic infection in terms of both protective immunity and immunopathology. Of particular interest is recent evidence that different parasitic infections in the context of different host genetic background can trigger polarized CD4+ T cell subset responses. The set of cytokines produced by these different T helper responses, in turn, can have opposing effects on the parasite, resulting in either control of infection or promotion of disease. Moreover, cytokines produced by one CD4+ subset can block either the production and/or activity of the cytokines produced by the other subset. The establishment of this state of cross-regulation may be important for parasite survival. CD8+ T cells also appear to play a dual effector/regulatory role in parasite immunity and immunopathology, although the mechanisms underlying their induction and function are less well understood. CD(8+)-mediated cytolytic killing functions have now been demonstrated against a number of different intracellular protozoa, although IFN-gamma produced by the same effector cells may also be critical in host community. In addition to providing highly relevant models for studying the selection and immunobiologic function of T-cell subsets, research on T lymphocyte-parasite interactions is crucial for the design of effective vaccines and immunotherapies and thus has broad practical as well as theoretical ramifications.