The traditional, fragmented approach to increasing manufacturing efficiency has resulted in "islands of automation" in our factories. Computer Integrated Manufacturing (CIM) is the goal of typing together these islands into a single coherent system capable of controlling an entire manufacturing operation. The technical and organizational difficulties of such a massive undertaking require a modular approach to CIM implementation, with an initial nucleus being gradually expanded by allowing interaction between it and other systems' databases. Manufacturing Resource Planning (MRP II) is best positioned to serve as this nucleus. The suggested first system for integration is Computer Aided Design (CAD); the integration being centered around part specification, product structure, and engineering changes. A functional model of the MRP II/CAD integrated system, detailing the logical interactions between the systems in these areas, is presented. In it, CAD serves as the center for design related decisions, while MRP II handles the task of executing and monitoring manufacturing activities. Integration is simulated using an interoperability system which allows for the specification of a set of rules to define and enforce the update and retrieval dependencies of the databases.