Photonic crystalsPhotonic Crystal (PC) (PCs)—nano-scale periodic air-hole lattice structures with a high refractive-index contrast—have attracted much attention to ultimate light-confinement and dynamic-control behaviors. This chapter surveys fundamentals of unique PC concepts such as lattice geometry, specific band structure, and peculiar optical phenomenon: The first topic is index contrast-based 1-D to 3-D PC structures and dispersion relations with band diagrams and full band gaps strongly dependent of lattice configuration. The second topic is significances of point/line defects specific to the PC and defect modes in the band gap. These concepts straightforwardly generate nano-scale cavities and waveguide devices. In particular, 2-D PC slab waveguides and their unique features such as mode confinement in the slab, light cone, and leaky mode are indispensable design guidelines for constructing an integrated PC platform. As the third topic, initial pioneering works on the theoretical and experimental demonstration of 2-D PC waveguides are described. The final topic is peculiar phenomena of the PC leading to unique applications, a super-prism effect based on the extraordinary refraction, an extremely high-Q nano-cavity, a slow-light effect resulting from the low group velocity, and a coherent surface emission at a Γ-point as an origin of the 2-D PC surface-emitting laser.