The payload carried by the X-Ray Multi-Mirror Mission (XMM), the second Cor
nerstone of the ESA Horizon 2000 Science Programme, consists of three scien
tific instruments: the Reflection Grating Spectrometer (RGS), the European
Photon Imaging Camera (EPIC), and the Optical Monitor (OM). This article pr
ovides a general overview of the main characteristics of all three instrume
nts.