A photonic crystal fiber (also called microstructure fiber, holey fiber, holeassisted fiber, or micro-structured optical fiber, etc.) is a single material optical fiber which obtains its waveguide properties from an arrangement of very tiny and closely spaced airholes which go through the whole length of the fiber. Unlike the traditional fiber, both the core and cladding are made from the same material in PCFs and light can be well confined and guided properly through the fiber by the mechanism of either total internal reflection (TIR) or photonic band gap (PBG). This book discusses the characteristics, performance and applications of photonic crystals. Chapter One reviews the design characteristics and optical properties. Chapter Two studies band structure of metal/dielectric photonic crystals. Chapter Three describes the splitting method in multicore photonic crystal fiber (PCF). Chapter Four focuses on switches, isolators, circulators, and multifunctional components for optical and THz regions based on 2D photonic crystals with magneto-optical resonators.