A graph-oriented object database model (GOOD) is introduced as a theoretical basis for database systems in which manipulation as well as conceptual representation of data is transparently graph-based. In the GOOD model, the scheme as well as the instance of an object database is represented by a graph, and the data manipulation is expressed by graph transformations. These graph transformations are described using five basic operations and a method construct, all with a natural semantics. The basic operations add and delete objects and edges in function of the matchings of a pattern. The expressiveness of the model in terms of object-oriented modeling and data manipulation power is investigated. Index terms: Database models, query languages, graph transformations, objectoriented databases, user interfaces. Preliminary versions of this paper were presented at the 9th ACM Symposium on Principles of Database Systems [16] and the 1990 ACM SIGMOD International Conference on Management of D...