The presence of even quite low levels of particles in drinking water is a matter of some concern, because of the possible presence of Cryptosporidium, which has been responsible for several recent outbreaks of waterborne disease. Turbidity measurements are by far the most common means of monitoring for particles in filtered water, but it is known that turbidity monitoring is rather insensitive for particles in the size range of Cryptosporidium oocysts (4-6 μm). The physical basis of turbidity by light transmission and light scattering is briefly reviewed, and alternative methods are considered. These include particle counting, which can give detailed information on the number and size of particles, and is highly sensitive for particles larger than about 1 μm. It is shown that a simpler technique, based on fluctuations in transmitted light, is very much more sensitive than turbidity for larger particles, and has some advantages over particle counting for routine monitoring of filtered water quality. La presence de concentrations meme tres basses de particules dans l'eau potable est preoccupante a cause de la presence possible de Cryptosporidium qui a ete responsable de plusieurs cas recents de maladies causees par l'eau. La mesure de turbidite est de loin le moyen le plus commun pour controler les particules dans l'eau filtree, mais on sait que la regulation par la turbidite est plutot insensible pour les particules dans la gamme de taille des oocytes de Cruyptosporidium (4-6 μm). Les bases physiques de la turbidimetrie par transmission de la lumiere et diffraction sont brievement rappelees et des methodes alternatives sont considerees. Celles-ci incluent le comptage des particules qui peut donner des information detaillees sur le nombre et la taille des particules et est tres sensible pour les particules plus grande que 1 μm. On montre qu'une technique plus simple, basee sur les fluctuations de la lumiere transmise, est beaucoup plus sensible que la turbidimetrie pour les particules plus grandes et possede quelques avantages sur le comptage des particules pour le controle de routine de la qualite de l'eau filtree.