We review the fundamental principles, components, functionalities, and applications of microwave photonics. This symbiotic field, which exploits the best of the radio frequency and photonic worlds, is expected to have a significant role in various scenarios where wireless and wired signals must coexist and be interfaced. This not only cover the future 5G telecommunications but also the emerging field of the Internet of Things and others such as biophotonics and broadband instrumentation.