Since the infamous outbreak in Milwaukee, WI, USA, of water-borne cryptosporidiosis affecting over 400,000 people, there have been at least 20 smaller outbreaks associated with this parasite in the UK and North America. These events have led to an explosion of interest in and research on the nature of cryptosporidiosis as a dangerous water-borne pathogen, particularly patients with acquired immune deficiency syndrome (AIDS). In addition, several major environmental laws and proposed regulations specifically address the control of this parasite. The possible ramifications of these laws include billions of dollars of modifications to water-treatment facilities in the USA. Unfortunately, the methods used to gather the information on which these laws are based have serious deficiencies that could lead to gross underestimation of the magnitude of this problem. The present review considers gaps in our understanding of water-borne cryptosporidiosis, new methods under investigation that could improve our ability to monitor water for the presence of this organism, and treatment and control strategies to limit the threat to our water supplies.