AbstractInfection with Giardia intestinalis is one of the most common parasitic infections among humans. In many parts of the world ∼20% of the population is infected with Giardia at any given time, and most children show evidence of having been infected by the age of 3 years. The majority of these cases are controlled within 2-3 weeks by host immune responses. However, chronic infections can develop in otherwise healthy patients. Variability also exists in the clinical presentation of Giardia infection. Some individuals experience severe cramps, nausea and malabsorptive diarrhoea while infected, while others have no obvious symptoms. Nutrient malabsorption and changes in enterocyte activity may still occur in the absence of overt symptoms. Understanding the roles of host immune responses and parasite genetics in this variability in duration and presentation of Giardia infection are thus major challenges to our ability to deal with this disease. This chapter focuses on the role of immune responses in contributing to both parasite elimination and disease pathogenesis in this infection.