Ceramics are by definition inorganic non-metallic materials, formed from a powder and by firing converted to a permanent solid mass. Firing is thus a sine qua non for the creation of a ceramic product. This chapter discusses the mineralogical changes and the changes in physical properties which occur in raw materials during the firing process. Experimental firing (in the laboratory and in the field) is a method used to examine the relationship between the properties of ceramic products and raw materials as well as firing conditions. Re-firing, as opposed to experimental firing, involves the firing of ancient ceramic fragments in the laboratory. This procedure enables ancient ceramics to be classified by the type of plastic raw material used in their manufacture and also provides an estimation of equivalent original firing temperature.