Decreasing the energy consumption of future wireless communication networks and improving their energy efficiency is demanding ever-growing attention around the world. Making ICT (information and communications technology) equipment and applications "greener" in terms of energy consumption and electromagnetic radiation will not only help telecommunication companies to attain their sustainable profitability, but will also have a profound positive impact on the global environment and human society. This article accordingly introduces the basic concept and features of green radio, and the relevant fundamental solutions for increasing energy efficiency at the network as well as the component levels. Moreover, the key role of cognitive green radio has been strengthened for improving energy efficiency, within which several representative learning algorithms, decision-making techniques, and design tools are thoroughly discussed, with power-consumption constraints. Finally, the electromagnetic-pollution issuein the scenario of opportunistic spectrum access (OSA) is also investigated.