We propose an epistemic dynamic logic EDL able to represent the interactions between action and knowledge that are fundamental to planning under partial observability. EDL enables us to represent 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 domain, such as determinism, full observability, unobservability, or pure sensing. Plan verification corresponds to checking the validity of a given EDL formula. The allowed plans are conditional, and a key point of our framework is that a plan is meaningful if and only if the branching conditions bear on the knowledge of the agent only, and not on the real world (to which that agent may not have access); this leads us to consider ``plans that reason'' which may contain branching conditions referring to implicit knowledge to be evaluated at execution time.