Abstract: Most object-oriented methodologies assign to instance models a minor role, using them to depict behavior scenarios and to show examples. For a large number of applications, among which enterprise models represent an outstanding example, traditional class-based models are not sufficient as tools. An enhanced object-oriented modeling methodology, has to be devised in order to face problems with using instance models to describe large and complex systems, such as enterprises. In addition CASE tools are needed, which provide a valid support to the construction of instance models .In particular the conformance of an instance model to a class model must be guaranteed. The OPJ approach is presented which tries to solve both organization and tool-oriented problems.