Prior to the advent of 3D treatment planning, a radiation therapy treatment planning system (RTPS) operated in near isolation. There were few, if any, interconnections between the planning system and other information systems in the institution. Today, the RTPS operates as one node in a continuum of information systems and processes that can include imaging systems and archives, information systems, other RTPS nodes and systems, and delivery and delivery verification systems. In order to integrate a new RTPS into the departmental process, the planning for purchase of an RTPS must go far beyond the traditional comparisons of contouring, dose calculation algorithms, and dose evaluation tools. This presentation will attempt to guide the medical physicist in planning the purchase of a new RTPS. Issues such as system infrastructure, network architecture, integration planning, and continuing training will be discussed. The medical physicist must consider the changes planned at their institution, such as 4D delivery, adaptive radiation therapy, and new treatment techniques. Each of these will influence the direction that the RTPS purchase should take. RTPS features can also force changes in the treatment planning process, as differences in areas such as image registration, plan normalization, and optimization may require technique and quality assurance changes in order to effect an orderly and safe transition. Finally, an upgrade strategy should be included in the purchase decision, so that future improvements in the RTPS can be effectively integrated into the overall treatment planning and delivery process. Educational Objectives: 1. To describe the overall planning process for the purchase of a radiation therapy planning system. 2. To identify key steps in the purchase process for a radiation therapy planning system under the aegis of the medical physicist. © 2007, American Association of Physicists in Medicine. All rights reserved.