The substantial increase of brilliance produced by new dedicated synchrotron sources calls for equivalent progress in the performance
of beam-line instrumentation. One of the basic aims is to obtain more precise focusing while collecting more photons. In particular, considering both the multiple functions of x-ray mirrors as beam-line components and the various characteristics of insertion devices as x-ray sources, a best compromise must be found among three concurrent requirements: versatility, high quality, and stability under severe working conditions. The author presents an overview of optical, thermal, and mechanical problems encountered with the design and realization of x-ray mirrors for third-generation synchrotron sources. The case of the European Synchrotron Radiation Facility is discussed to illustrate the specific requirements when dealing with the new hard x-ray insertion devices.