Highly efficient and environmentally friendly direct converters of chemical into electrical energy fuel cells promise an important role in future power supply. There exist several types of fuel cells in the temperature range from ambient temperature up to about 1000°C. This entry considers particularly the high-temperature solid oxide fuel cells (SOFC) which besides power, also deliver heat and are therefore interesting for domestic applications. But SOFCs have also been under development as auxiliary power units (APUs) in cars and trucks. The principle, function, design, and structure of SOFCs are described together with the quality demands which the cell components have to meet. Different technologies and techniques, partially in competition or complementary, are applied to produce the cells. In particular, plasma technologies using direct current (DC) and radio frequency (RF) plasma sources are discussed. Information about the development state of plasma-produced SOFCs are given in comparison with such made by other methods. The limitations of the applied plasma processes and the demands and potentials for further improvement to get a favorite role for plasma technologies for mass production of SOFCs are also discussed. Due to their close relationship to SOFCs with solid oxide electrolyzer cells (SOEC), which function in a way opposite to SOFCs, i.e., heat and power resulting in chemical energy (hydrogen or syngas) some remarks about them can be found at the end of the entry.