In presence of incomplete information about the world we need to distinguish between the state of the world and the state of the agent's knowledge about the world. In such a case the agent may need to have at its disposal sensing actions that change its state of knowledge about the world and may need to construct more general plans consisting of sensing actions and conditional statements to achieve its goal. In this paper we first develop a high level action description language that allows specification of sensing actions and their effects in its domain description and allows queries with conditional plans. We give provably correct translations of domain description in our language to axioms in first-order logic and logic programming, and relate our formulation to several earlier formulations in the literature. We then analyze the state-space of our formulation and develop several sound approximations that have much smaller state-spaces. 1 Introduction and Motivation Unlike actions t..