Abstract: The first Martian mission of the European Space Agency, Mars Express, was launched on 2 June 2003. During its normal scientific phase, a low frequency topside radio wave sounder (MARSIS) on board Mars Express probed the Martian ionosphere and collected a vast amount of data. We provide a brief review of the fundamental principles of Martian ionosphere detection based on MARSIS, the structure of the data, and current research results obtained from the data.