A generic model for the internal dynamic behaviour of BDI agents is proposed. This model, a refinement of a generic agent model, explicitly specifies beliefs and motivational attitudes such as desires, goals, intentions, commitments, and plans, and their relations. A formal meta-language is used to represent beliefs, motivational attitudes and strategies. Dynamic aspects of reasoning about and revision of beliefs and motivational attitudes are modelled in a compositional manner within the modelling framework DESIRE.