This chapter reviews various spectrum-sharing methods that increasingly are being used to respond to the escalating demand for spectrum, which has been sparked by the seemingly unstoppable surge in new wireless services and technologies. Spectrum sharing encompasses several techniques – some administrative, some technical, and some market-based. Spectrum can be shared in several dimensions: time, space and geography. Limiting transmission power is also a way to permit sharing among low-power devices operating in the spectrum “commons” – as explained later with dynamic spectrum access, which takes advantage of power and interference reduction techniques. Sharing can also be accomplished through licensing and/or commercial arrangements involving spectrum leasing and trading.