and groups. Other extragalactic, extended X-ray sources are given by the inverse Compton scattering from relativistic jets in radio galaxies, from supernova remnants, hot gas and massive X-ray binaries in star-forming galaxies, and low-mass X-ray binaries in normal galaxies. In this paper, we mainly focus on galaxy clusters and groups.

Thanks to the development of tools for analyzing the morphology of extended sources and the capability of simultaneous imaging and spectroscopy with CCD, a wealth of new physical phenomena has been opened to direct investigation in the field of galaxy groups and clusters. The most noticeable discoveries include the interaction between the ICM and the relativistic jets of the central radio galaxy, which is directly observed as bubbles inflated by the jets; the presence of cold fronts; and the physics of cool-cores and their metal distribution. However, less attention has been paid to the detection of very faint extended sources, despite the increasing interests in distant groups and clusters of galaxies. Based on the serendipitous discoveries of high-z massive clusters of galaxies through X-ray observation of [*XMM-Newton*]{} and [*Chandra*]{} or through the Sunyaev-Zeldovich effect [see @jee09; @foley11; @santos11], we expect a significant population of massive clusters at very high redshift 