Giardiasis is a major diarrheal disease found ubiquitesly. The flagellate protozoan Giardia lamblia (other names are G intestinalis or G duodenalis), its causative agent, is the most commonly identified intestinal parasitic pathogen and the most common protozoal intestinal parasite isolated worldwide. Infection is more common in children than in adult patients. G. lamblia can cause asymptomatic colonization or acute or chronic diarrhea. The parasite has been found in as many as 80% of raw water supplies from lakes, streams, and ponds and in as many as 15% of filtered water samples. It is a commonly responsible for chronic diarrhea and growth retardation in children in developing countries. Giardiasis usually shows a zoonosis with cross-infectivity between animals and humans. Giardia lamblia has been isolated from the stools of beavers, dogs, cats, and primates. Beavers are an important reservoir host for G lamblia. Other Giardia species are G muris in rodents; G agilis in amphibians; G psittaci and G ardeae in birds; and G microti in voles and muskrats. Giardia species are endemic in areas of the world that practice poor sanitation measures. In developing countries like India, the disease is an important cause of morbidity. Water-borne and food-borne outbreaks are commonly seen. Because ingestion of as few as 10 Giardia cysts can cause infection, giardiasis is common in daycare center attendees and institutionalized patients in developed countries. G lamblia is a particularly important pathogen for people with malnutrition, immunodeficiencies, or cystic fibrosis. High-risk groups for giardiasis include travelers who are travelling in highly endemic areas, immunocompromised individuals, and sexually active homosexual men. Cyst passage rates as high as 20% have been seen in certain groups of sexually active homosexual men; these individuals were frequently symptomatic. The traditional basis of diagnosis is identification of Giardia lamblia trophozoites or cysts in the stool of infected patients. Stool antigen enzyme-linked immunosorbent assays also are available. Standard treatment for giardiasis consists of Metronidazole, tinidazole, and nitazoxanide which are the drugs of choice. Metronidazole is the most commonly prescribed antibiotic for this condition.