This paper introduces the concept of robustness and illustrates one way in which it could be quantified by means of an example involving filtration in relation to Cryptosporidium removal. With regard to a serious water contamination incident that occurred in Canada during May 2000, the robustness of each of the five elements (source, treatment, distribution, monitoring and response) is assessed qualitatively to explain the overall vulnerability of the water supply system in that town. Finally, robustness concepts are described as part of a process to invest in new capital facilities for regulatory compliance. Includes 12 references.