Formal Development of Correct Classes in Computational Logic Kung-Kiu Lau Department of Computer Science, University of Manchester Oxford Road, Manchester M13 9PL, United Kingdom kung-kiu@cs.man.ac.uk Chris Moss Department of Computing, Imperial College 180 Queen's Gate, London SW7 2BZ, United Kingdom cdsm@doc.ic.ac.uk Mario Ornaghi Dipartimento di Scienze dell'Informazione Universita' degli studi di Milano Via Comelico 39/41, Milano, Italy ornaghi@dsi.unimi.it Abstract: Quality software must be reusable, extensible and reliable. In computational logic , we have developed an approach to constructing programs that are formally correct. Our approach can provide a basis for constructing software that is (formally) reusable and extensible, and not just reliable but formally correct. In this paper, we explain our notion of correct classes, and how to develop them.