Giardia is a ubiquitous and well-known enteric parasite affecting humans and a range of domestic and wild mammals. It is one of the most common parasites of domestic dogs and dairy cattle and a frequently recognized waterborne pathogen. Giardiasis is considered to be a re-emerging infection because of its association with outbreaks of diarrhoea in child-care centres. Although only a single species has been recognized as causing disease in humans and most other mammals, molecular characterization of morphologically identical isolates from humans and numerous other species of mammals has confirmed the heterogeneity of this parasite and provided a basis for a clearer understanding of the taxonomy and zoonotic potential of Giardia.