Radiative cooling (RC) is a sustainable technology that passively lowers the temperature of terrestrial objects through thermal radiation, dissipating heat into outer space via the atmospheric window, unlike conventional cooling systems that require electricity and release waste heat to the environment. In recent years, RC technologies have advanced rapidly, moving steadily toward practical implementation and commercialization. Here, this mega-review offers a comprehensive overview of RC, encompassing its fundamental background, the underlying photonic and thermal principles, and fabrication approaches for constructing efficient cooling devices. We discuss a variety of RC structures along with their design strategies and explore the diverse functionalities that adapt RC toward particular requirements. We further review emerging applications of RC, highlighting practical implementations. Finally, we outline the key challenges and future directions, offering perspectives and insights to guide continued progress in this rapidly advancing field.