Numerous investigations during the last few decades have clearly demonstrated the existence of a wide array of immune mechanisms in teleosts. An increasing amount of evidence indicates that both non-specific and specific factors in the humoral and cellular fish immune systems are well developed. However, a successful co-evolution of the host and its parasite necessitates that the latter develop evading mechanisms in order to avoid extinction. Some parasites have even understood to benefit from the well developed antiparasitic armament in fish epithelia. Thus, a range of parasites are exploiting the antiparasitic response mechanisms of the host to optimize host-finding, invasion and survival in the host. Such interactions between hosts and parasites are phylogenetically old. These dynamic interactions between hosts and invaders have been observed in various branches of fish parasitology. Experiments with protists such as Ichthyophthirius multifiliis and monogeneans such as Gyrodactylus derjavini indicate that parasites are activated by factors in host mucus (first line of defence) and actively search for epithelia rich in mucous cells. Some monogeneans, cestodes, digeneans, acanthocephalans and crustaceans are able to resist pronounced cellular host reactions which even will improve the attachment of the parasites to the host tissue. Despite these evading mechanisms in the parasites it is possible to use immunoprophylactic measures to control infections. Some parasites are indeed actively rejected by their potential hosts provided these are effectively immunized at certain early points before infection.