Transparent conducting oxides (TCOs) have been used in several applications for three or four decades but, despite the huge volume of experience in the field, there remain many unanswered questions at both applied and fundamental levels. The reason that these issues have not been addressed, or only superficially so, is largely because the performance of the TCOs has been adequate to meet the demands of most applications considered thus far. For the purposes of the present document, the applications of concern are photovoltaic cells and modules, but the broader range of applications includes transparent electrode materials for both electrochromic cells and for liquid crystal display devices. In the case of both solar cells and the other two applications, improvement in performance of the TCOs is important because their non-ideal properties will eventually impact the performance of the complete device. We shall provide a definition of the term performance later in this document. From this point, we shall address the specific demands on TCOs imposed by prospective photovoltaic applications. In the field of polycrystalline thin film solar cells, the dominant materials at present are copper indium gallium diselenide [1] (CIGS) and cadmium telluride [2] (CdTe). In addition, a-Si:H cells depend sensitively on the properties of the TCO transparent2 electrode. The first of the above cells is generally known as a substrate device and second as a superstrate device. The practical implication of this distinction is that the TCO element is either deposited directly onto the glass substrate before the absorber layers are deposited, or it is deposited on top of the semiconductor stack after all of the layers have been deposited. Although it is obvious that specific optical properties are required, additional important criteria must be met by the TCO and by the method used for its deposition. We shall discuss these later.