Applications of PC waveguides to advanced photonic networks are overviewed here by selecting four typical ones. Their origins are introduced in Chap. 2 as selected topics on the PC waveguides, i.e., slow light in the defect-mode PC band, fabrication of low-loss air-bridge PC waveguides, extremely high-Q 2-D PC nano-cavity, and coherent surface emission at the Γ-point in the 2-D PC. The first slow-light PC application to signal processing includes a tunable delay/dispersion device, symbol-rate variable differential quadrature phase-shift keying (DQPSK) receiver, etc. The second ultra-fast all-optical switch application includes multi-functional key configuration of symmetrical MZI-based 2-D PC device modified with large optical nonlinearity of quantum dots (QDs) embedded in the PC. The third optical nonlinearity-based bi-stable device application includes a high-Q nano-cavity in the 2-D PC waveguide for integrated optical DRAMs. The final innovative 2-D PC surface-emitting laser (PCSEL) application includes various excellent characteristics; unprecedented tens-of-watt class high power operation using 3 mm-emission area with single mode and high beam-quality. Unique applications to compact and high-resolution 3-D LiDAR are also reported using new types of PCSELs such as double-hole lattice and dually modulated PCSELs.