[en] The pressurized light water cooled, medium (1000 MWt) power plant IRIS (International Reactor Innovative and Secure) has been under development for four years by an international consortium of over 20 organizations from ten countries. The plant conceptual design was completed in 2001 and the preliminary design is nearing completion. The pre-application licensing process with NRC started in October, 2002 and IRIS is one of the designs considered by US utilities as part of the ESP (Early Site Permit) process. IRIS is a pressurized water reactor that utilizes an integral reactor coolant system layout. Its containment is only a fraction of the size of corresponding loop reactors, resulting in a significant reduction in the overall size of the reactor plant. The IRIS reactor vessel houses not only the nuclear fuel and control rods, but also all the major reactor coolant system components including pumps, steam generators, and pressurizer. The pressurizer is located in the upper head, above the internal control rod mechanisms and, in the current design has an overall volume of about 80 m³ (about 2800 ft³). The current configuration is very convenient since it minimizes the dimensions of the vessel, operates with both of the closure flanges at a uniform temperature, and maximizes the overall pressurizer volume, while providing adequate space for placements of the reactor coolant pumps and internal CRDMs. The pressurizer design and its main features are discussed in this paper. Since the early design stages, when the option for a steam pressurizer without active spray was made, there has been an increasing number of performance requirements set for the pressurizer. Examples of these are the elimination of power operated relief valves and the constraint to reduce the number of events that could cause the opening of the pressurizer safety valves. Several internal design documents have been prepared to properly define the functional requirements as well as the means to assure compliance with them. As a result, more detailed and sophisticated models have to be developed to cope with this new level of refined analysis. This paper summarizes the most relevant functional requirements and their means of implementation, describes some features of the models used for pressurizer analysis, and presents and comments on some of the results obtained so far.(author)