Knowledge base (KB) is an important component knowledge-based systems (KBS). Due to the fact that KB is often built in an incremental and piecemeal fashion, potential errors may be inadvertently brought into it. KB verification refers to the process of ascertaining the correctness of a KB, which is an integral part of the effort to guarantee the overall correctness of a KBS. KB verification presents a problem of multiple dimensions. Though many verification tools and techniques have been developed or proposed, no coordinated effort has been made in establishing a standard for evaluation and comparison of those tools. It is our hope that the issues raised below and the summary of the existing approaches may serve as a steppingstone toward the convergence of a unified thoery on KB verification.