Giardiasis is the most prevalent intestinal parasitic disease in the United States. Most cases can be diagnosed by a single stool examination. However, in periodic cyst excretors, cysts may not be detected unless repeated stool examinations are performed. In clinically highly suspected cases, duodenal fluid should be studies after three negative stool examinations. Scanning electron microscopy is probably superior to light microscopy in screening duodenal fluid if the parasite is scanty or degenerated. A small intestinal biopsy is a last resort for the diagnosis; a mucosal impression smear should be routinely performed on such specimens. The latter is the most sensitive and reliable technique in making the diagnosis.