Dramatic advances in research and development on photonic crystal fibers (PCF) have created new properties that had not been achievable using conventional fibers. PCFs have many fascinating features. The very high relative refractive index between the fiber core and air-hole cladding enables several prominent properties, notably endlessly single-mode (ESM) operation, high nonlinearity, wide-ranging dispersion management, and the ability to maintain high polarization. The processes involved in manufacturing PCFs are quite different from those used to manufacture conventional fibers, and this is largely because of the profusion of air holes in the silica glass that comprises PCFs. The authors have optimized the technology to manufacture photonic crystals with the required optical characteristics. This paper describes the properties and application of large-mode and ESM guidance, nonlinearity, and double cladding. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.