The high throughput x-ray spectroscopy mission XMM is the second cornerstone project in the European Space Agency (ESA) long-term program for space science. This observatory has at its heart three large x-ray telescopes, which will provide a large collecting area with a spatial resolution better than 15 arcsec. In 1998, the three Flight Models and the two spare models of the x-ray telescope have been delivered to ESA, after verification and calibration at the Centre Spatial de Liege and at the Max Planck Institute near Munich. They show mechanical and optical performances much better than the specification. Their performance will undoubtedly bring an important benefit for the astronomers. The most challenging part of the development of these telescopes was the design, the manufacturing and the testing of the x-ray mirrors, during four intensive years. In this paper, we will first summarize the development and the excellent results of the mirrors. Then, we will discuss the lessons learned during this development, mainly in terms of management and technical aspects, including design, manufacturing and testing. These lessons will be drawn in perspective of the future x-ray missions such as Constellation X, Xeus and ASTRO-F.