The XMM catalogue provides an effective dataset for generating large, well-defined samples of various types of astrophysical object, notably active galaxies (AGN), clusters of galaxies, interacting compact binaries and active stellar coronae, using the power of X-ray selection. The large sky area covered by the serendipitous survey also means that XMM is a rich resource for exploring the variety of the X-ray source populations and identifying rare source types. Not all public observations are included in the catalogues. The XCATDB interface is design to process complex queries including multiple constraints on cross-correlated couterparts.
