Transparent conducting oxides (TCOs) are increasingly critical components in photovoltaic cells, low-a windows, flat panel displays, electrochromic devices, and flexible electronics. The conventional TCOs, such as Sn-doped In2O3, are crystalline single phase materials. Here, we report on In-Zn-O (IZO), a compositionally tunable amorphous TCO with some significantly improved properties. Compositionally graded thin film samples were deposited by co-sputtering from separate In2O3 and ZnO targets onto glass substrates at 100 C. For the metals composition range of 55-84 cation% indium, the as-deposited IZO thin films are amorphous, smooth (R-RMS < 0.4 nm), conductive (sigma 3000 Omega(-1) . cm(-1)), and transparent in the visible (T-Vis > 90%). Furthermore, the amorphous IZO thin films demonstrate remarkable functional and structural stability with respect to heating up to 600 degrees C in either air or argon. Hence, though not completely understood at present, these amorphous materials constitute a new class of fundamentally interesting and technologically important high performance transparent conductors.