A computer implemented method of combining two knowledge bases, each knowledge base comprising concepts that are linked by relations, the method comprising: assigning one of the knowledge bases as a first knowledge base and the other of said knowledge bases as an additional knowledge base; matching concepts between the first knowledge base and the additional knowledge base to define mapping relations between concepts of the first and additional knowledge base; assessing defined mapping relations to determine if they cause a violation with relations already present in the first or second knowledge base; modifying relations within the additional knowledge base to repair violations; and storing an extended first knowledge base comprising the first knowledge base, the defined mapping relations and the additional knowledge base with the modified relations within the additional knowledge base.