The COLUMBUS Attached Pressurized Module (APM) is the European orbiting laboratory which will be permanently attached to the International Space Station Freedom (SSF).It is designed to provide a range of laboratory facilities in a microgravity environment for payload experiments originating from the international payload-user community.The individual payloads will in general be mounted in payload racks which can be accommodated in fixed positions on the left and right hand sides of the laboratory and in the ceiling. International standard payload racks (ISPR) can be located in any of the SSF laboratory elements and find compatible interface conditions subject to agreements made between the international partners (NASA, NASDA and ESA).The APM design provides a water cooling capability by means of moderate temperature (MT) and low temperature (LT) pumped fluid loops. The cooling loops serve both the APM essential subsystem equipment as well as the payload users. The APM Thermal Control Subsystem has design responsibility for the fluid loops up to the payload rack interfaces. Each fixed payload rack position is provided with fluid connectors for connection of the rack to the cooling loop. Payload racks can be changed out on-orbit and can be located at any position offering the required resources.This paper describes the architecture of the APM water cooling loops and the water cooling capability offered to the payload users. Thermal and hydraulic requirements applicable to the payloads are identified. A number of rack internal cooling options are examined. These options ensure the necessary heat removal while avoiding any possible failure propagation or contamination of the subsystem loops. The resulting constraints and limitations on the payload design needed to ensure compatibility of the payload with the fluid loops and their operational conditions are identified.