This chapter primarily considers the properties of ceramics used in low-temperature cofired ceramics circuits, high temperature cofired ceramic circuits, and the more standard ceramics, including aluminum oxide, beryllium oxide, and aluminum nitride. Ceramics are the foundation of many microelectronic circuits, acting as the substrate to deposit conductive, resistive, and dielectric films to form interconnections and passive components. There are two major surface properties of interest, surface roughness and camber, both highly dependent on the particle size and method of processing. In ceramics, the heat flow is primarily due to phonon generation, and the thermal conductivity is generally lower than that of metals. The mechanical properties of ceramic materials are strongly influenced by the strong interatomic bonds that prevail. A force applied to a ceramic substrate in a tangential direction may produce tensile or compressive forces. The electrical properties of ceramic substrates perform an important task in the operation of electronic circuits.