Abstract: Modern photoelectronic products and energy technologies largely exploit transparent conducting oxide (TCO) thin films. Due to the increasing demand for solar cells, flat-panel displays, light emitting devices, short-wavelength lasers, energy-conserving windows and so on, TCO thin films are finding more and more applications. This article analyses and summarizes the operating principles, application demands, and current research directions of TCO. Emphasis is placed on the key problems of p-type TCO thin films, including doping asymmetry, performance degradation, and the generation, structure and development of defects. The critical factors regarding preparation of TCO thin films, leading-edge research, and promising opportunities for achieving breakthroughs are pointed out.