The life cycle of XMM-Newton's 'Targets of Opportunity'

The term 'Target of Opportunity' (ToO) is used in astronomy to identify unp redictable events whose study is of the highest scientific interest. For XM M-Newton a ToO is an astronomical event observable by its instruments, whic h cannot be predicted and scheduled on the time scale of one year, yet is s cientifically significant enough to justify the interruption of the ongoing observing programme. Here we discuss the kinds of objects that are suitabl e for observation as ToOs with XMM-Newton, their scientific interest, and h ow they are incorporated into the overall observing schedule in terms of ta rget selection and mission planning. What has been done so far and the prel iminary results for a few ToO examples are then presented.