XMM-Newton has been flying for more than 21 years and has made almost a million X-ray detections. Amongst these objects, many are rare types of compact object systems, often identified through their strong variability, some times thanks to the long baseline over which a source can be observed. Here we present some of the transients discovered in the XMM- Newton catalogues, produced by the XMM-Newton Survey Science Centre (XMM-SSC) and describe software that is being put into place to detect, identify and communicate serendipitous transients in quasi-real time, to allow them to be followed-up rapidly.