We propose an integration of a fragment or propositional dynamic logic with an epistemic logic so as to represent logically the interactions between action and knowledge that are fundamental to planning order incomplete knowledge and partial observability. This logic, called EDL, enables us representing incomplete knowledge, nondeterministic actions, observations, sensing actions and conditional plans; it also enables a logical expression of several frequently made assumptions about the nature of the problem such as determinism, full observability, unobservability, or pure knowledge gathering. Plan verification corresponds to testing the validity of a given formula of this logic. The allowed plans are conditional, and a key point of our framework is that a plan is meaningful if the branching conditions bear on the knowledge of the agent only, and not on the real world (to which tha agent may not have access); this leads us considering “plans that reason” which may contain branching conditions referring to implicit knowledge to be evaluated at execution time.