During the past years, we have witnessed an impressive growth in the demand for online video streaming applications. Examples are on-demand services, online media centers, teleconferencing services, and social networks. As a global consequence, studies reported that in 2018, 1% of the world-wide greenhouse gas emissions were caused by online video applications, with yearly growth rates close to 10%. Hence, research targeting the energy efficiency and the sustainable use of online video technology is of growing importance in the fight against climate change. This lecture provides an overview on the current knowledge and the state of the art in research on the energy consumption of online video applications. First, the major sources for greenhouse gas emissions are discussed, which are the production of devices, the energy demand of data centers, of the transmission, and of end-user devices. Afterwards, we take a detailed look on systems performing streaming tasks such as encoding, decoding, and rendering. In particular, the energy efficiency of end-user devices such as smartphones or tablet PCs will be discussed. Finally, an outlook is presented highlighting open problems and upcoming research directions.