The structure and composition of several different types of glazes are examined in order to understand the physical basis of their appearances and some of the underlying technology which produced them. Cultural constraints on appearance are also discussed. Important differences between glaze and glass are mentioned. The physical basis of visual appearance is based on the interaction of the ceramic body and its glaze with light. This is also discussed. The types of glazes are reviewed and presented in a historical framework for the technological change and development of glazes. Technological changes in glazing are shown to be gradual. And, each glaze-producing craft tradition is shown to be the result of a combination of choice and limitation of materials, cultural constraints, and experimentation and innovations of the craftsmen.