This paper discusses the results to date of a continuing study which seeks to answer questions which have been raised regarding the comparability of methods used for determining the viability and/or infectivity of Cryptosporidium oocysts. Most of these questions derive from the highly variable and often conflicting results of published viability/infectivity studies. For example, some studies have compared surrogate measurements of viability such as vital dye staining to in vitro excystation while others have compared them to neonatal mouse infectivity assays based on different mouse strains using various inoculation techniques and data analysis methods. This situation has made it very difficult for the water industry to determine which viability measurement methods are best. The two specific aims of the study are: to determine how reliably each of these methods correlates with the others in determining viability/infectivity, and to demonstrate the usefulness of each surrogate in practice by examining different types of disinfection processes and the use of fresh versus environmentally stressed (aged) oocysts.