Recently, a new type of solid oxide fuel cells has been developed employing extremely thin oxide electrolyte. These fuel cells are expected to operate at significantly reduced temperature compared to conventional solid oxide fuel cells. Accordingly, they may resolve the stability and material selection issues of high temperature fuel cells. Furthermore, they may eliminate the limitations of polymer membrane fuel cells whose operation temperature is under $100^{circ}C$. In this paper, we review the electrolytes for intermediate temperature operation. Then, we discuss the current development of thin film solid oxide fuel cells that possibly operated at low temperatures.