Despite rapid progress in understanding the biology of Giardia, several questions remain unanswered. First, there is no adequate explanation for the diverse clinical spectrum of giardiasis. Second, the mechanisms by which Giardia produces diarrhea and malabsorption are poorly understood, although some progress has been made. Finally, despite extensive studies in animal models and human infections, the key immunologic determinants for clearance of acute infection and development of protective immunity remain ill defined. This article discusses the epidemiology, pathology, diagnosis, treatment, and prevention of giardiasis.