This text aims to be a reference for the practitioners and academicians in the fields of multimedia search engines. Half a terabyte or 9000 hours of motion pictures are produced around the world every year. Furthermore, 3000 television stations broadcasting for twenty-four hours a day produce eight million hours per year, amounting to 24,000 terabytes of data. Although some of the data is labelled at the time of production, an enormous portion remains unindexed. For practical access to such huge amounts of data, there is a great need to develop efficient tools for browsing and retrieving content of interest, so that producers and end users can quickly locate specific video sequences in this ocean of audio-visual data. This text describes the main techniques being developed by the major players in industry and academic research to address this problem.