Radio spectrum refers to the range of frequencies used for various communication services, including mobile voice and data communications, TV, radio, Wi-Fi, amongst many. The allocation and management of spectrum is crucial because it is a limited natural resource. Wireless communications is often compared to automobile traffic, where different spectrum bands and channels act as the roadways and lanes. Just as in this analogous scenario, rules and regulations are necessary to ensure that the traffic across different lanes is managed efficiently, particularly when there is high demand competing for a limited number of available lanes. This chapter introduces classification of radio spectrum, regulatory bodies managing the spectrum, and technical bodies such as ITU-R and 3GPP that define the rules of spectrum use. It also explains important radio access network aspects such as power levels within specified bands and channels, out of band emissions and the nature of restrictions put in place to ensure coexistence between various RF systems.