Several recent trends have changed the usage and users of schemas beyond those of a database administrator’s tool for describing database contents. Distribution of computing power to the desk-top and increasing needs for data management have broadened the base of schema users to include people who are not database experts. The advent of graphical user interfaces has brought schemas into use as templates for a variety of database operations such as query specification and browsing. Such changes demand advanced schema management techniques, primarily schema visualization, in order to support productive interaction between increasingly novice users and increasingly complex schemas. In this paper, we present OPOSSUM, a flexible, customizable, and extensible schema management system. Working within the established paradigm of schema editing through direct manipulation, OPOSSUM employs several novel techniques to offer the following capabilities: enhancement of schema visualizations with user-specific information; exploration of schemas through choice of visual representations; and creation of new visual representation styles when existing ones prove unsatisfactory. We discuss the architecture of the system and the methodology that guided its development, and illustrate its most important features through examples of how it has been used. OPOSSUM is operational and is in use by three groups of experimental scientists on the University of Wisconsin campus as a tool for experiment and database design.