Giardia lamblia is discussed as the most important parasitic cause of travellers' diarrhoea. The parasitology of this flagellate protozoan is discussed with respect to route of infection, threshold infecting dose, and incubation period. The geographic distribution of giardiasis is widespread, being common in tropical and sub-tropical regions and endemic at a low level in European and North American countries. The predisposing factors are detailed together with clinical manifestations, pathogenesis and diagnosis. Effective therapy may be quinacrine t.i.d. for 5-10 days, metronidazole 200 mg t.i.d. for 14 days or 2 g once daily over 3 days, or a single dose of 1.5 g of tinidazole. Although Entamoeba histolytica is distributed world-wide, it rarely causes travellers' diarrhoea. However, the clinical features, diagnosis and treatment of amoebiasis-related travellers' diarrhoea are detailed.