Photonic crystals are 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 occur in nature and in various forms have been studied scientifically for the last 100 years. This book gathers and presents topical data in the field of photonic crystals including the phenomena in photonic crystals stipulating the presence of omnidirectional band gaps, i.e., overlapping of stop bands in all directions; the physics of photonic crystal couplers and their applications; photonic crystals and their capability to control the propagation and emission of electromagnetic waves and others.