Parasitology is traditionally the study of eucaryotic parasites. It encompasses protozoa (unicellular animals) and helminths (worms), which are sometimes collectively called the macroparasites, as opposed to the microparasites (bacteria and viruses). This inherently fascinating and practical subject will be presented as a compact survey of the major clinically important protozoa and helminths. This chapter covers elements of their morphology, life cycles, epidemiology, pathogenic mechanisms, and methods of control. Arthropods, which are often included in the field of parasitology, have been previously discussed as vectors for certain viral and bacterial infections in chapters 13, 20, and 21. Other chapters containing information on parasites are chapter 5 (eucaryotes), chapter 7 (nutrition), and chapter 12 (chemotherapy).