The main purpose of the paper is to introduce philosophers and philosophical logicians to dynamic logic, a subject which promises to be of interest also to philosophy. A new completeness result involving both after — and during — operators is announced.

The modal logician's notion of possible world and the computer scientist's notion of state of a machine provide a point of commonality which can form the foundation of a logic of action. Extending ordinary modal logic with the calculus of binary relations leads to a very natural logic for describing the behavior of computer programs.

In the paper we define a class of languages for representation o knowledge in those application areas when a complete information about a domain is not available. In the languages we introduce modal operators determined by accessibility relations depending on parameters.