A solid oxide fuel cell (SOFC) electrochemically converts chemical energy of a fuel into electricity at temperatures from about 650 to 1000oC. SOFCs offer certain advantages over lower temperature fuel cells, notably ability to use CO as a fuel rather than being poisoned by it, and high grade exhaust heat for combined heat and power, or combined cycle gas turbine applications. This paper reviews the operating principle, materials for different cell and stack components, cell designs, and applications of SOFCs. Among different designs of solid oxide fuel cells (SOFCs), the electrical resistance of tubular SOFCs is high, and areal power density (W/cm2) and volumetric power density (W/cm3) low. Planar SOFCs, in contrast, are capable of achieving very high power densities.