In this talk, I shall begin by presenting some generalities about transparent conducting oxides (TCOs), including work at NREL, their typical properties and their relevance to solar cells. I shall demonstrate how a badly selected TCO can severely impact the performance of a new generation of high performance thin-film solar cells. I then demonstrate that the electron mobility of TCOs is the key property to which all investigations ought to focus. Only via high mobility can the dual requirements of excellent optical transmittance and electrical conductivity be achieved. Following this analysis, I apply the Drude theory to demonstrate that fundamental properties (such as the electron effective mass and relaxation time) of TCOs may be determined from their optical properties. Next, we discuss the use of advanced characterization methods, including the method of four coefficients which may be used to determine effective mass, relaxation time, Fermi energy, and also to provide information about the charge scattering process that is limiting the mobility of the electrons. I then illustrate the value of this technique by showing results for cadmium oxide and zinc oxide and show how a picture of the electron structure of these materials may be obtained. Finally, if time permits, I shall then discuss our work at NREL on a novel TCO system, Mo-doped indium oxide. This has demonstrated excellent mobilities using sputter deposition and points to a new class of TCO films.