Over the past five years, the Philadelphia Water Department has expended significantresources to better understand the challenges that Cryptosporidium poses to its customers. These efforts have included the following:routine influent and effluent monitoring for all of its water treatment plants to determine baselineoccurrence and levels in the water supply; special watershed-wide and storm event monitoring studies to better understand the sources, fate and transport, and influencing factors on raw water levels; studies of removal andamounts in wastewater discharge; various studies to determine the potential removal of protozoa by water treatment; epidemiological monitoring of endemic cryptosporidiosis in the community and communication plans with local and state health agencies; and investigations of swimming pool exposure. This paper discusses the results of these studies. Includes table, figures.