The selection of the substrate material is dependent on the application for which the deposited circuit is intended. The normal production specifications for substrates are discussed and a brief description given of the production methods available. The reasons for the impurity ratio in the ceramic must be considered relevant to the type of deposition that will be applied and the degree of adherence that is required; there is also a connection between impurity level and surface finish which will affect the choice of metallizing media. In terms of both cost and surface texture, ‘as fired’ ceramics are favoured. Any subsequent machining operations will tend to expose voids which can be clearly seen on roughness traces, though a general ‘flattening’ of crystal peaks will take place. If a higher degree of surface finish is obligatory, then great care is needed in the selection of ceramic with particular reference to the size of grain and alumina content. Finally, the factors affecting the costs of production substrates are considered in light of the aspects discussed.