An overview is given of a light-weight, Object-Oriented (OO), multi-database system, named AMOS II. Object-Oriented multi-database queries and views can be defined where external data sources of different kinds are translated through AMOS II and integrated through its OO mediation primitives. Through its multi-database facilities many distributed AMOS II systems can interoperate. Since most data reside in the data sources, and to achieve good performance, the system is designed as a main-memory DBMS having a storage manager, query optimizer, transactions, client-server interface, etc. The AMOS II data manager is optimized for main-memory and is extensible so that new data types and query operators can be added or implemented in some external programming language. Such extensibility is essential for data integration. 1 Introduction The AMOS II system has its roots in the workstation version of the Iris system, WS-Iris , and the DAPLEX  functional data model. The core of AMOS II..