We present a first-order formalism for dealing with agents’ cognitive attitudes in a dynamic setting. We first extend our ontology in order to represent agents’ beliefs and goals. These mental attitudes are expressed in the situation calculus by means of accessibility fluents that represent accessibility relations among alternative situations. Then, we consider changes of mental attitudes in a dynamic and incompletely specified world. Changes may be caused either by the evolution of the external world or by the acquisition of new information. In particular, acquisition of information that modify agents’ cognitive attitudes is expressed by cognitive actions. The effects of cognitive actions are characterized by suitable axioms, thus providing a model for the evolution of the alternative situations and the accessibility fluents. We discuss our proposal and compare our model of change with the characterization of Belief Revision postulated by Gärdenfors. We finally introduce the problem of describing agents in a dynamic environment, and briefly sketch a possible extension of the theory that copes with this problem.