The Relationlog system is a novel persistent deductive database system for advanced data and knowledge-based applications. It directly supports the storage and inference of data with complex structures, especially data supported in nested relational and complex-object models. The Relationlog system supports the Relationlog query language, which is a typed extension of Datalog with tuples and sets and stands in the same relationship to the nested relational and complex-object models as Datalog stands to the relational model. It also supports an SQL-like data definition language and a declarative data manipulation language. This article introduces the Relationlog language, discusses the system architecture, the design decisions incorporated within its implementation, and our experience in developing the system. Copyright © 2001 John Wiley & Sons, Ltd.