This document sets out the characteristics of instruments used for X-ray powder diffraction (“powder” as defined in EN 13925-1:2003, Clause 5) as a basis for their control and hence quality assurance of the measurements made by this technique. Performance testing indicators are given for diffractometer performance testing. Different types and makes of X-ray powder diffractometer vary considerably in their design and intended fields of application. This document attempts to cover as much of this range as possible by keeping to common principles. To make the standard more readily applicable, the Bragg-Brentano configuration is addressed in most detail because of its wide use. Additional considerations and adaptations may be necessary to cover some types of instruments or configuration and some fields of application. Some of these types of instrument are described in Annex B.