Zinc-indium-tin oxide (ZITO) is a potential replacement for the currently used tin-doped indium oxide (ITO) as a transparent conducting oxide (TCO) for optoelectronic devices. At the present time ITO is the material of choice for the TCO layer, but the increasing cost of indium metal and the advent of new technologies will require alternative TCOs. Over the past 15 years, bulk and thin film studies have been amassed that report the electrical and optical properties of various ZITO compositions. This review will examine the reported data and demonstrate that the bulk subsolidus phase diagram can act as a guide to understanding the numerous and varied results reported for thin films.