Parasitic diseases are a major constraint to efficient livestock production throughout many parts of the world. For the last 50-60 years controlling these diseases has relied, to a large extent, on the use of anti-parasitic agents. However, for some diseases there are no effective anti-parasitic drugs whilst, for others, the relatively late onset of clinical symptoms and diagnosis following infection means that chemotherapy is only partially effective. Conversely, parasites that replicate, re-cycle and transmit rapidly through host populations, such as ticks, mites, enteric nematodes and avian coccidia, require more-or-less continuous mass administration of drugs via food, water or spraying. These latter types of chemoprophylactic control regimes impose enormous selective pressure on the parasite populations, resulting in widespread emergence of drug-resistant organisms.