The problem of Cryptosporidium in water is briefly reviewed, with some mention of the effectiveness of available treatment methods. Since conventional disinfection can be much less effective than for other pathogens, it is essential to achieve very high degrees of removal by physical means. Cryptosporidium oocysts are nearly spherical, with a diameter of around 5 mum, and should be no more difficult to remove than other particles of similar properties. Nevertheless, conventional treatment such as flocculation followed by rapid filtration through granular media can allow some oocysts to pass. Specific monitoring methods for Cryptosporidium are available, but these are time-consuming and subject to some uncertainties. They are not suitable for on-line monitoring. Alternative, non-specific monitoring techniques are considered, including turbidity and particle monitoring. It is shown that particles in the oocyst size range are not sensitively detected by conventional turbidity monitoring, and that a very low treated water turbidity may not imply the absence of Cryptosporidium. Similar remarks apply to Giardia cysts. Particle counting instruments or a simpler particle monitor based on turbidity fluctuations, may provide a more reliable indication of the presence of particles a few microns in size, including Cryptosporidium and Giardia. The basic principles of these techniques are reviewed, and some results of laboratory testing and trials in water treatment plants are given.