Photonic crystal is a versatile platform for the design and development of optical devices used in high-speed computing and telecommunication networking systems, which is due to the bandgap engineering that a photonic crystal offers and also due to its ability to control the light in nanometer scale. Design of photonic crystal-based all-optical logic gates with simple structures paves a way toward high-speed data processing applications due to its unique features. In this chapter, a simple photonic crystal-based T-shaped waveguide is presented and analyzed using a coupled mode theory to represent its functional ability as a logic gate. Consideration of wide-angled branches of the waveguide as the inputs of the logic gate leads to provide higher contrast ratio of above 50 dB. The high contrast ratio allows these gates to interconnect or cascade with each other to realize complex logic functions. Thus, the T-shaped waveguide-based logic gates are interconnected in single- and double-stage structures to observe the complex functionality. Apart from the T-shaped waveguide, L-bend and straight waveguides described in this chapter are useful in the multistage or multigate functional design.