We review the recent technology advancements related to the transmission of 400Gb/s, wavelength-division-multiplexed channels for optical networks based on the standard 50 GHz grid. We discuss the enabling modulation, coding, and line system technologies, as well as the existing challenges. Specifically, these technologies include time-domain hybrid 32-64 quadrature amplitude modulation, nearly ideal digital Nyquist pulse shaping, improved channel bandwidth management methods such as end-to-end carrier frequency control and distributed compensation of filtering effects arising from reconfigurable optical add/drop multiplexers, distributed Raman amplification, and powerful forward error correction. We demonstrate 400G transmission on the standard 50 GHz grid over meaningful transmission reach for regional and metropolitan applications. However, further studies are needed to fully understand the potential for meeting the requirements of long-haul transmission applications.