We emphasize in this chapter the Sol-Gel chemistry, which has gained a large number of researchers, developing interesting and sophisticated novel synthetic methods, offering a variety of approaches to new systems preparation, overcoming many of the synthetic difficulties of the past. A strong argument for using the Sol-Gel chemistry is found in the high flexibility of the method and the large choice of commercially available “dopants” that can be incorporated in the solid matrices, that might have a specific activity or reactivity to an external signal (i.e. light, magnetic, electrical, etc). From the point of view of nanotechnology applications, Sol-Gel materials are being required for critical components embedded in systems such as industrial equipment and scientific instrumentation, imaging and display, medical applications, aerospace and defense, etc. The rapidly developing sol-gel process has been used for the preparation of materials for a wide range of fields, adapting the chemistry and the novel synthetic routes to the specific systems, in order to achieve complicated developments oriented to nanotechnology applications. Clear examples can be found on Sol-Gel optics applications.