In 1978 Kenneth Hill of the Communications Research Centre in Ottawa discovered that intense blue light propagating inside the core of a germanium-doped silicate glass fiber modified the core refractive index sufficiently to form a measurable permanent hologram. Because germanium-doped silica is the material of choice for the core of most optical fiber in use today for optical communications, this “photosensitive” phenomenon has been recognized as having tremendous practical importance. Fiber Bragg gratings in particular form excellent narrow-band optical filters with a multitude of applications: sensors, fiber laser mirrors, wavelength multiplexers (the acronym for wavelength multiplexing and demultiplexing in systems is WDM), and dispersion control devices—to name a few that are already commercially available. The importance of this field can be verified in just about any current issue of journals related to lightwave technology and applications of optics. Fiber Bragg gratings are mentioned everywhere. In fact a recent issue of the Journal of Lightwave Technology is entirely devoted to this topic (and to poling of silica, a field reviewed by W. Margulis in this issue).