XMM is the second comprehensive catalogue of serendipitous X-ray sources from the European Space Agency's (ESA) XMM-Newton observatory. The 2XMM catalogue is the largest X-ray source catalogue ever produced, containing almost twice as many discrete sources as either the ROSAT survey or pointed catalogues. 2XMM complements deeper Chandra and XMM-Newton small area surveys, probing a much larger sky area.