There are a number of design and operational issues associated with the implementation of ultraviolet (UV) disinfection in water treatment plants. The major issue is the measurement of the applied UV dose. Due to a very short detention time in the reactor, validation of distribution of UV dose in the entire reactor is another issue facing the industry. Design issues consist of reliability, redundancy, number and types of lamps, monitoring requirements, number of sensors, type of cleaning and its frequency, impact of water quality parameters, and system instrumentation and controls. The issues are complicated by the fact that UV lamp radiation is variable and depends on a number of factors such as temperature, voltage and lamp age. In water treatment, unlike chemical disinfectants, applied UV dose cannot be reliably measured with currently available instrumentation. Distribution of UV dose is also impacted by the hydraulic flow characteristics within reactors with different lamp configurations. The American Water System (AWS), the largest and most geographically diverse water service provider in the nation, owns and operates more than 80 surface water treatment plants and each plant is a possible candidate for application of UV technology. Therefore, a project was initiated to evaluate design and operational issues associated with the current state of UV technology. As part of the project, inactivation kinetics of Giardia and Cryptosporidium and other indicator microorganisms are being developed in laboratory experiments. A full-scale UV reactor will be operated for a period of one year to identify and develop solutions to design and operational issues. Biodosimetry experiments using the indicator microorganisms will be used to determine the applied UV dose in the full-scale reactor. Includes 14 references, table, figures.