The development of database technology has currently reached the stage of deductive database systems which use Horn clauses for defining relations. An important characteristic of these systems is the clear separation of logic and control. However, the programmer cannot affect the control part of a deductive database system. To eliminate this deficiency, this monograph presents a so-called expert deductive database system that allows explicit control of the deduction process. The system consists of an object-level describing the logical aspects of a problem and of a meta-level that contains application-specific control information affecting the object-level deduction process. For example, object-level rules can be disregarded, and some tuples deduced at the object-level can be preferred to others. Besides the architecture of this system, the book also identifies some important possibilities of deduction control which are explained by characteristic examples.