Cryptosporidiosis was recognised in human beings in 1976, and was prominent in the 1980s and 1990s as a cause of severe diarrhoeal illness in patients with AIDS. It is now additionally recognised as a major cause of waterborne diarrhoeal illness in developed regions, and as a pathogen with long-term effect on childhood growth and development in impoverished areas. This update focuses on recent changes in our understanding of the taxonomy of cryptosporidium, its epidemiology, effects, pathogenesis, diagnosis, and treatment.