This paper addresses verification issues in combining knowledge bases or theories from different sources. Tile knowledge may be acquired using direct knowledge acquisition or inductive learning methods. Keeping the consistency of knowledge becomes all important issue when integrating two or more knowledge bases. Several independently consistent knowledge bases may give an inconsistent system when they are combined. The inconsistencies which could be syntactical or semantic are described in this paper and some suggestions to handle them are given. This process can be used to refine knowledge bases, in terms of validity and completeness, and to obtain a larger integrated system.