Over the past two years Calgon Carbon has validated several large Ultraviolet (UV) Drinking Water Disinfection Reactors at flow rates of 1 to 40 MGD at the UV validation facility in Portland, Oregon. The UV Reactors utilize medium pressure mercury arc lamps in a closed vessel with diameters ranging from 18 inches to 48 inches. Validation involves the use of surrogate organisms, in this case MS2 Phage, that are added to the water stream ahead of the UV reactor along with agents that decrease the UV transmittance of the water thereby simulating waters of different quality. Effluent samples are collected at different operating conditions of UV Transmittance, Flow, Lamp power and Number of lamps. The reduction in viable organisms is used to obtain a UV Dose that can be correlated with the various operating parameters to generate operating curves for the reactor at a Water Treatment Plant (WTP). The use of independently traceable process measurements, such UV Irradiance at each lamp, is used to ensure that the reactor is providing adequate removal of the target organism such as Cryptosporidium, thus protecting public health. This paper discusses the design and validation as it pertains to the recently released draft US Environmental Protection Agency UV Disinfection Guidance Manual (UVDGM). Several factors are described including the methodology used to obtain validation equations, the use of safety factors to cover any error in validation and operation of reactors, and the ability to control the lamp power in a UV reactor to obtain the exact dose needed for the required disinfection credits. Includes 4 references, table, figures.