Data stored in a Picture archive and communication system (PACS) must be organized to permit efficient retrieval. The concept of a unique data object identifier (UID) permits a fundamental partitioning of the problem into a storage system, indexed by UID, and a database containing descriptive elements. The database serves to map user retrieval requests, expressed in terms of clinically relevant descriptive elements, and into UIDs of specific data objects. Different data organization mechanisms are employed by imaging modalities, thereby making the structure of a generic PACS database complex. One solution may be derived by analogy from film-based systems. Images and other data objects may be organized, by application of modality and site specific rules, into electronic folders. Folders may then be organized into a patient master folder or user-defined reference folders. Such an organization provides an easily understood user access model for information stored in PACS archives. This report presents a PACS architecture comprised of an Information Management System (IMS) and Information Storage System (ISS). Entity-relationship diagrams are presented to define a schema for the IMS database, based on the folder analogy. The folder concept and its relationship to the ACR-NEMA Standard are discussed.