Si photonics has significantly progressed recently owing to small size, low cost, and high power efficiency, all brought about by SOI CMOS process compatibility. Applications of Si photonics began in the data center and core network in the photonic network society. They are now expanding markets to new areas in sensing mobility and artificial neural network in the Internet of Things (IoT)IoTs (Internet of Things) society. This chapter begins with progress of Si photonics platform and then introduces latest applications to optical transceivers in the data centers and node switches in the core networks. Early activities of Si photonics platform are focused on heterogeneously integrated nano-lasers and hybrid nano-lasers between III-V compound and silicon materials using chip/wafer bonding and hetero-epitaxy. New topics on the sub-wavelength pitch Si nano-structures are introduced also. Topics on the optical transceiver are focused on three types; a monolithically integrated Si photonic transceiver using front-end SOI CMOS processes, a fingertip-size compact optical I/O core transceiver mounted on FPGAs, and a Si photonic transceiver for the 5G mobile network. Topics on the optical switch include a multi-cast Si photonic transponder aggregator (TPA) switch for ROADM with polarization-independent high practicality, and a multi-port (point-to-point) Si PILOSS optical switch for data-com markets. Challenges for further improvements in bandwidth, optical coupling, polarization dependence, etc., are discussed together with a possible higher (~ ns) switching speed using an EO (electro-optic) modulator. Novel applications to light detection and ranging (LiDAR) and optical neural network (ONN) are introduced in Chap.  5 .