The objective of this chapter is to present discussion on parasitic infections. Parasitic infection should be considered as potential cause for diarrhea lasting for more than 7 days in those whom other infectious or other noninfectious causes have been excluded. Amebiasis can cause hemorrhagic colitis and may occur several years after acquiring infection from endemic areas. Symptoms may be chronic, and may mimic those of inflammatory bowel disease. Definitive diagnosis is established by antigen test. Antigen test kits are available for routine laboratory testing of Entamoeba histolytica directly from stool specimens. Metronidazole is the mainstay of therapy for invasive amebiasis. Giardia lamblia is the most common parasite infection worldwide; the second most common is pinworm. Clinical manifestations vary from asymptomatic to watery diarrhea with abdominal cramping, nausea, vomiting and, rarely, failure-to-thrive. First-line therapy is metronidazole for 7–10 days. Other active agents include tinidazole, nitazoxanide, mebendazole and albendazole. Cryptosporidiosis, leishmaniasis, and toxoplasmosis are the other parasitic infections which are also covered thoroughly in this chapter.