Protozoans cause a number of important infectious diseases. This chapter discusses malaria, babesiosis, toxoplasmosis, trichomoniasis, leishmaniasis, and trypanosomiasis; in addition, the chapter describes the intestinal protozoan infections caused by Giardia lamblia; Dientamoeba fragilis; Entamoeba histolytica; Blastocystis hominis; the coccidial organisms Cystoisospora belli, Cryptosporidium, and Cyclospora cayetanensis; Balantidium coli; and microsporidia. Figures illustrate the taxonomy of pathogenic protozoans; the life cycle of malaria; identification of species of malaria based on forms seen on blood smears; Babesia parasites in erythrocytes; the life cycle, tachyzoites, and tissue cysts ofToxoplasma gondii; G. lamblia trophozoites and cysts; a D. fragilis trophozoite; E. histolytica cyst and trophozoite forms; Entamoeba coli; clinical involvement in amebiasis; B. hominis; cysts of Cryptosporidium, Cyclospora, and Cystoisospora; Acanthamoeba polyphaga cyst and histopathologic features; histopathology of Naeglaria meningoencephalitis;Leishmania (Viannia) braziliensis cutaneous infection; amastigotes of Old World leishmaniasis; the insect vector for Chagas disease; Romaña sign in acute Chagas disease;Trypanosoma cruzi, and T. brucei. Tables list the differentiating features of Plasmodium species that cause malaria; dosages, principal side effects, and main limitations of antimalarial drugs; and forms of New World leishmaniasis. A sidebar provides Internet links on protozoan infection. This review contains 22 highly rendered figures, 3 tables, and 121 references.