[en] The prototype version of the ZEUS Expert System, ZEXP, was diagnosing selected aspects of DAQ System during ZEUS running in 1993. In November 1993 ZEUS decided to extend its scope in order to cover all crucial aspects of operating the ZEUS detector (Run Control, Slow Control, Data Acquisition performance and Data Quality Monitoring). The paper summarizes fundamental assumptions concerning the design of the final version of the ZEUS Expert System, ZEX. Although the theoretical background material relates primarily to ZEX, its elements can be used for constructing other expert systems for HEP experiments. (orig.)