This article studies the importance, requirements, and trends of synchronization for radio communications. Fifth-generation (5G) wireless communication is expected to be a forthcoming revolution in the wireless world, achieving communications with a high peak rate, high spectral efficiency, multiple device connectivity, low end-to-end latency, and high reliability. From a historical perspective, we review the synchronization required for first-generation (1G) to fourth-generation (4G) mobile communications to achieve performance metrics and provide various new services. New synchronization specifications and standards may depend on specific new applications and are, therefore, more service oriented. This article surveys the progress and trends of synchronization requirements in standardizations and specifications.