This book comprehensively introduces photonic crystals and their applications including design and modeling characteristics. Photonic crystals are described as the periodic optical nanostructures that are designed to affect the motion of photons in a similar way that periodicity of a semiconductor crystal affects the motion of electrons. Photonic crystals are described as attractive optical materials for the purpose of manipulating and controlling the flow of light. Specifically, photonic crystals are of significant interest for basic as well as applied research, and two dimensional photonic crystals are being fast employed to commercial applications like micro electro-mechanical systems (MEMS), optical logic devices, and sensors. The first commercial products involving two-dimensionally periodic photonic crystals already exist in the form of photonic-crystal fibers, which employ a microscale structure to limit light with radically distinct features in comparison to traditional optical fiber for applications in guiding wavelengths and nonlinear devices.