The low-altitude satellite DEMETER was in operation for more than 6 years, from June 2004 until December 2010. Its main scientific objective was to establish if there are ionospheric perturbations prior to earthquakes. Its scientific payload allowed the measurement of waves at various frequencies and of plasma parameters at the satellite altitude. The aim of this chapter is to present the main results obtained from the data analysis. It includes a short description of the satellite, its experiments and the data processing centre. To date, more than 130 papers have been published with the DEMETER data to show that seismic processes can influence the ionosphere not only at the time of the earthquakes but also hours and days before. Other examples of these ionospheric perturbations are shown in the chapter, together with the results of several statistical analyses. At the end, the problem of earthquake prediction is discussed.