Giardia lamblia, the protozoan parasite, first described by Von Leewenhoek in 1681, has come into prominence in last quarter century because of mounting awareness that it may cause significant morbidity and loss of man power. Earlier thought to be a commensal organism, it has been recognised as a true intestinal pathogen in the past three decades. Nevertheless, the mechanism of the disease caused by this protozoan parasite (in the human host's small intestine) continues to remain unexplained. The infection with G. lamblia is worldwide with an average prevalence of 12.5 per cent and is especially common in children and may cause failure of child to thrive. The G. lamblia infection has been implicated in a number of water borne epidemics and is important cause of traveller's diarrhoea all over the world. Infection with G. lamblia may be entirely asymptomatic, may produce a mild, self limiting illness or chronic diarrhoea with or without malabsorption. The reasons for such variations in severity are not clearly understood. However, interplay of virulence of parasite, nutritional status and type of the host immune responses and its effect on intestinal mucosa appear to modulate the infection.