In this paper we study the dynamics of belief from an agent-oriented, semantics-based point of view. In a formal framework used to specify and to analyze rational agents, we define actions that model three well-known changes of belief, viz. expansions, contractions and revisions. We define both the opportunity for and the result of these belief-changing actions. To define the semantics of the contraction action we introduce selection functions. These functions pick out a set of states that is to be added to the set of doxastic alternatives of an agent, thereby contracting its set of beliefs. The action that models belief revisions is defined as the sequential composition of a contraction and an expansion in a way suggested by the Levi-identity. We show that these belief-changing actions are defined in an intuitively acceptable, reasonable way by proving that the AGM postulates for belief changes are validated.