A review of excimer laser systems is presented, including basic physical data, technology, performances and applications. The last few years have seen a rapid development of this new type of gas lasers and of many commercial systems producing efficient high power pulses of ultra-violet radiation. After a short recall of physical principles, we give a description of the internal structure of nanosecond and picosecond excimer lasers, followed by the analysis of principal beam characteristics : output power, efficiency, pulse stability, beam profile and lifetime. Some cost considerations are also examined. The presentation of principal applications in various domains as photochemistry, material processing, non linear processes and medecine, shows how these lasers are definitely an useful tool.