The relationships between parasites and hosts are complex, with many of these interactions involving an amazing degree of biochemical coevolution and communication. Hormones, neurohormones, and growth factors figure prominently in these relationships. In vertebrate hosts, many parasites secrete hormones, neuropeptides, or cytokine-like molecules that influence the host's physiological and immunological responses. Alternatively, the parasites secrete factors that alter the host's hormone levels. Simultaneously, molecules emanating from the host strongly influence the parasites' success. In some cases the host's hormones directly influence the parasites; in others, effects are mediated indirectly via the host's immune system. In invertebrates, the presence of parasites likewise has a major influence on the host's endocrine status and the normal suite of processes governed by hormones, including host development, metamorphosis, and reproduction. In insects, interactions involving juvenile hormone and ecdysteroids are especially well-documented, and recent evidence suggests that neuropeptides may also be affected by parasitism. Moreover, recent data suggest that in some species, such as snails, the host's nervous, neuroendocrine, and immune systems are functionally linked, similar to the complex interactions seen in vertebrates.