The Catskill and Delaware watersheds supply New York City with up to 90% of its daily water demand of 1.2 billion gallons of water. Due to the high raw water quality of these unfiltered supplies and an extensive watershed protection program, the New York City Department of Environmental Protection (NYCDEP) has been granted a Filtration Avoidance Determination (FAD) by the US Environmental Protection Agency (USEPA). As part of the FAD, NYCDEP must proceed with the design and installation of ultraviolet (UV) disinfection facilities. The purpose of this paper is to review the selection of a lamp technology for the UV facility and discuss the unique series of challenges that had to be addressed in the process. The design of a UV disinfection facility for New York City's Catskill and Delaware supplies poses several unique design challenges due to the scale of the facility (2,020 mgd), the properties of the unfiltered surface water sources, gravity flow system, and the relative infancy of UV technology in the U.S. drinking water industry, as regulations continue to evolve. A major challenge has been to identify manufacturers capable of producing a UV system with a unit capacity of 40 mgd, a size that has never been built before. During the development of the UV facility design concept, both low pressure high output (LPHO) and medium pressure (MP) systems were evaluated and deemed feasible for providing the disinfection level needed to meet the anticipated regulations for inactivation of Cryptosporidium. Considerable research and evaluation was conducted on the relative merits and drawbacks of the two systems, including a series of meetings with potential manufacturers, canvassing of other users, and site visits to large operating facilities. While no 40-mgd UV unit has ever been constructed, units capable of treating about 20 mgd for microbial inactivation are now in service. The paper details the primary features of LPHO and MP systems, outlines their primary advantages and disadvantages, and presents how New York City arrived at the decision to proceed with the LPHO lamp technology for its Catskill/Delaware UV disinfection facility. Includes tables, figures.