Preface: The 5th International Workshop on X-ray Mirror Design, Fabrication, and Metrology.

Recent developments in synchrotron storage rings and free-electron laser-based x-ray sources with ever-increasing brightness and coherent flux have pushed x-ray optics requirements to new frontiers. This Special Topic gathers a set of articles derived from a subset of the key presentations of the International Workshop on X-ray Mirrors Fabrication (IWXM-2015) and Metrology held at Lawrence Berkley National Laboratory, Berkeley, California, USA, July 14-16, 2015. The workshop objective was to report on recent progress in x-ray synchrotron radiation mirrors fabrication as well as on new developments in related metrology tools and methods.