Giardia lamblia are protozoan parasites which cause human intestinal disease. The life cycle has a multiplying intraduodenal trophozoite and an excreted cyst. Infection occurs after cyst ingestion from faecally contaminated water or by direct faecal-oral transmission in situations of poor sanitary standards, but the zoonotic nature of giardiasis is debated. The pathophysiology may arise from enzyme or active transport deficiencies, synergy with intestinal bacteria or an immunopathological process. Diagnosis is made by microscopic identification of cysts or trophozoites in small bowel samples or faeces. Symptoms are acute with diarrhoea (without blood), abdominal cramps, bloating and flatulence. The treatment of choice is either metronidazole or tinidazole. No vaccine or drug prophylaxis exists, and measures to avoid cyst ingestion should be undertaken.