Starting with a brief review of the photonic crystal concept, we describe fundamental features, fabrication technologies, experimental advances, and potential impact of photonic crystals. The progress achieved in photonic crystal research is closely connected to the evolution of device design concepts through a deeper understanding of nature of photonic bands. We describe detailed methods for the numerical and experimental determination of the photonic band structure that are useful and essential for analyzing photonic crystals as well as for understanding and engineering their unprecedented optical properties. This chapter closes with a discussion of the exciting possibilities of a photonic crystal in nonlinear light-matter interactions and their device applications.