For some years, specification of software and hardware systems has been influenced not only by algebraic methods but also by new developments in logic. These new developments in logic are partly based on the use of algorithmic techniques in deduction and proving methods, but are alsodue to new theoretical advances, to a great extent stimulated by computer science, which have led to new types of logic and new logical calculi. The new techniques, methods and tools from logic, combined with algebra-based ones, offer very powerful and useful tools for the computer scientist, which may soon become practical for commercial use, where, in particular, more powerful specification