There is currently intense interest in the use of ultraviolet technology for helping to meet future regulations relating to improved microbial inactivation and decreased disinfection byproducts (DBPs). Much of this interest has been fueled by recent research indicating the effectiveness of UV technologies for the inactivation of Cryptosporidium. This scientific discovery, combined with proposed regulations concerning DBPs in the United States, has resulted in a need to evaluate the feasibility and effectiveness of UV irradiation for use as a disinfectant for drinking water. The objectives of this project were to: (1) develop and evaluate physical, chemical, and biological methods for calculating effective germicidal UV dose from medium-pressure (MP) and Pulsed-UV (P-UV) lamps; (2) establish a UV dose/log inactivation relationship for specific bacterial and viral indicators for MP and P-UV lamps; (3) determine the extent of photoreactivation and dark repair of heterotrophic bacteria after treatment by MP and P-UV lamps; and (4) compare the ability of MP and P-UV lamps to inactivate Cryptosporidium. UV dose measurement on the bench scale was achievable with all the methodologies tested. It was determined that the inactivation of Cryptosporidium parvum was rapid under all the UV lamp types tested. The inactivation of heterotrophic bacteria was also rapid and similar for all lamp types. Any recovery in growth of heterotrophic bacteria was suppressed with chloramine addition. It appeared that the inactivation of MS2 coliphage was appreciably improved when exposed to light from polychromatic, UV lamps as compared to the LP monochromatic UV lamp. Originally published by AwwaRF for its subscribers in 2003