Over the last decade, several countries in Europe started to accept the widespread use of ultraviolet (UV) systems for the disinfection of drinking water. The use of UV technology for that particular application in the U.S. is still relatively small in comparison. Only over the last 2 - 3 years has the focus of the North-American municipalities and designers shifted towards UV as an additive disinfection possibility. The most important precursor of the increased attention was the release of the US Environmental Protection Agency (USEPA) UV Disinfection Guidance Manual (UVDGM; 1st draft release in 06/2002 and 2nd draft release in 06/2003). With the USEPA officially preparing the ground of giving correctly sized UV systems inactivation credits for Cryptosporidium, Giardia and virus, UV becomes available for US-American utilities as an important disinfection tool with a series of benefits in regards to health and environmental friendliness. The UVDGM provides the requirements for a correctly sized UV system, the first of which is to undergo a validation process. This is required of every UV reactor model that a manufacturer wishes to use in the municipal drinking water market. Secondly, it needs to be operated within the boundaries of the successful validation. A big challenge to using UV in accordance to the UVDGM, is that a multitude of UV dose rates (or better: Reduction Equivalent Doses = REDs) are acceptable, depending on the actual target organism and log inactivation credit (project-specific sizing). Secondly, it does not specify one sole method to arrive at a specific UV dose but allows three possibilities (validation-specific sizing). Finally, there are two approaches for arriving at a certain inactivation credit (validation/manufacturer-specific sizing). Overall, the possible variations and project-, customer- and manufacturer-specific requirements lead to a very complex design scenario. Includes 14 references, tables, figures.