As indicated in Chapter 1 , many of the earliest applications of the sol-gel method involved metal oxide materials. Significantly, these applications fell largely into two categories: decorative coatings and constructional materials. Today, while our im­ proved understanding of the sol-gel process has been applied to the development of new generations of ceramic materials, new applications of metal-oxide-based functional materials with a higher level of sophistication have greatly widened the scope of sol-gel technology. New catalysts made by the sol-gel method attract the attention of chemists and chemical engineers; new electronic materials accessible via the sol-gel route find wide application in electronic devices and displays; new metal oxide semiconducting sensor materials offer improved sensitivity and selectivity, providing a link between the chemical environment and the computer. In contrast to the historical applications of the method, these new applications demand a detailed understanding of both the principles of the applications area and the principles of the sol-gel method if successful synergies are to develop. In this chapter we illustrate this process with reference to ceramic materials, catalysts, electronic materials and sensor materials.