. We investigate the formal specification of the dynamic reasoning process of knowledge-based systems. The main contributions of the paper are: defining a formal framework for describing the dynamic reasoning behaviour of knowledge-based systems which unifies existing approaches; defining a semantics for the specification of the dynamic reasoning behaviour of a knowledgebased system within the states as algebra setting that overcomes several shortcomings of the existing approaches; and providing for the first time an axiomatization and proof theory of specification languages for knowledge-based systems. We achieve this by developing the logical language MLPM (Modal Logic of Predicate Modification). 1 INTRODUCTION The model of expertise as developed in the KADS-I and CommonKADS projects  has become a widely used framework for developing and describing knowledge-based systems (KBS). Such a model of expertise can be used to describe the reasoning process and the knowl..