Multilayers are the optical elements of choice in any situation where flux rather than resolution is desired. They can be tailored to optimize either reflectivity at fixed or variable energy, or heat resistance, or bandpass, or harmonic rejection. The present state of the art of x-ray multilayers is presented, and the possible applications are reviewed, including the use of multilayers as soft x-ray polarizers, large-passband elements for hard x-rays, and power filters. In the situation of very intense beams the increase of temperature can produce significant changes of reflectivity, which have been extensively studied at the microstructure level in some cases, such as W/C, W/Si, and Mo/Si. Existing and prospective solutions are detailed, including heat treatment prior to x-ray exposure, use of compound materials, efficient cooling, and modification of the electric field distribution by nonperiodic arrangements.