This paper outlines the storage requirements for Picture Archiving and Communications Systems (PACS) and strategies for meeting those requirements. A radiology archive, either film based or PACS, follows a layered model, with each layer characterized by a required access time and typical retention time. For this paper, a three layer model is considered. In the first layer are images from newly acquired exams awaiting interpretation, and images from recently interpreted exams which may be needed for reference as part of continuing patient management. In the middle layer are images from exams which, while not currently active, are likely to be needed for either the diagnosis of new exams or for reference. In the final layer are images which are not active and are least likely to be required to support diagnosis or patient management. In a film based environment these layers correspond to the active file room or reading areas, the main (2 year) file room, and the inactive file room. In this model, the PACS will be defined in terms of a hierarchy of 1 week, 1 or 2 years, and anything older. Image generation rates have been gathered from surveys of nine hospitals. These data have been used to derive "large" (600-700 bed) and "small" (300-500 bed) target environments which generate the capacity requirements for the layers. These, coupled with the access time requirements, are used to identify data storage technology with each layer.