This book describes fully embedded board level optical interconnect in detail including the fabrication of the thin-film VCSEL array, its characterization, thermal management, the fabrication of optical interconnection layer, and the integration of devices on a flexible waveguide film. All the optical components are buried within electrical PCB layers in a fully embedded board level optical interconnect. Therefore, we can save foot prints on the top real estate of the PCB and relieve packaging difficulty reduced by separating fabrication processes. To realize fully embedded board level optical interconnects, many stumbling blocks need to be addressed such as thin-film transmitter and detector, thermal management, process compatibility, reliability, cost effective fabrication process, and easy integration. The material presented eventually will relieve such concerns and make the integration of optical interconnection highly feasible. The hybrid integration of the optical interconnection layer and electrical layers is ongoing.