This guideline deals with reflective X-ray optical systems, which use total reflection as well as Bragg reflection on inner interfaces of a multilayer structure to influence the spectral composition and the directional characteristic. The physical principles and determining parameters of these X-ray mirrors are described. The role and influence of substrate and coatings are explained. Total reflection mirrors and multilayer mirrors for various applications are discussed. The most important X-ray optical systems using X-ray mirrors (Kirkpatrick-Baez, Wolter, Schwarzschild, Montel und EUVL optics) are introduced briefly.