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High Altitude Platform Stations (HAPs) is a new and promising technology which can serve a large number of users at low-cost and quick access to modern wireless communication services. This paper reviews on the system structure, the advantages of HAPs over terrestrial and satellite architectures and possible communications applications of HAPs. It also present an overview about some underway HAPs projects worldwide. Finally, it focuses on the technical challenges and critical issues of energy source, platform station keeping, modulation, coding, antennas design, propagation, diversity, interference and handoff issues.