Although giardiasis has been a threat to mankind for thousands of years, this protozoan infection was, until recently, relatively neglected. Giardia duodenalis is recognised as a major cause of parasite-induced diarrhoea in humans and animals, and is currently an important public health problem, placing a heavy burden on both diagnostic and treatment services at health care institutions, mostly in developing countries, but also in highly developed countries. Steady progress in recent years, using a combination of molecular, immunological, and clinical approaches, has substantially increased our understanding of Giardia and important aspects of the clinical manifestations that it causes. The purpose of this review is to provide an overview of the extent of Giardia infection, the implications of water and food in Giardia transmission, new aspects regarding clinical diagnosis and environmental detection, treatment, and some approaches towards prevention and control. A number of future research priorities are also presented.