The many species of coccidia affecting domestic animals and man cause a variety of diseases which require extensive use of anticoccidial drugs to relieve economic losses or human suffering. We have had effective drugs to prevent or treat coccidiosis of most types since the late 1940s, but new drugs are constantly needed to replace those made useless by drug resistance. The approach to chemotherapy in domestic animals varies with the type of husbandry practiced. Young chickens are reared in large flocks and are at great risk from the several species of coccidia which have short, direct life cycles and great reproductive potential. Today anticoccidial drugs are mixed into poultry feeds at low levels for continuous prevention of coccidiosis, allowing coccidiosis control to be managed from a single point in large production units, the feed mill. In animals that are subject to sporadic exposure, medication is given only to treat clinically ill animals. Treatment of toxoplasmosis in humans is difficult and often will not prevent the severe side effects of the disease. Until recently there was no known treatment for cryptosporidiosis in man, and patients with a depressed immune response were likely to die from dehydration caused by the extreme diarrhea. New drugs are finally offering hope for cure of crytosporodiosis.