Nowadays, opto-electronic devices, such as displays, are omnipresent in our daily life. A crucial component of these devices is a transparent electrode, which allows the in- and out-coupling of light. With the goal of optimizing the electrode characteristics and improving device efficiencies, many approaches for the fabrication of thin, transparent conducting films have been studied. This review gives an overview of the different material classes which have been used as transparent electrodes, ranging from metal oxides, such as Indium Tin Oxide, metal and carbonaceous nanostructures, to conducting polymers and composites. For every material class a brief description of the fundamental principles, processing routes and the latest achievements is given. Furthermore, the different electrodes are compared regarding their opto-electronic performance, flexibility and surface roughness. Ultimately, advantages and drawbacks of the respective electrodes are discussed. This critical comparison of fundamentally different transparent conducting materials allows, on one hand, to make a sensible choice of electrode for specific applications, and, on the other hand, to point out scientific challenges that must still be addressed.Read less <