The term "Target of Opportunity" (ToO) is used in astronomy to identify unpredictable events whose study is of the highest scientific interest. For XMM-Newton a ToO is an astronomical event observable by its instruments, which cannot be predicted and scheduled on the time scale of one year, yet is scientifically significant enough to justify the interruption of the ongoing observing programme. Here we discuss the kinds of objects that are suitable for observation as ToOs with XMM-Newton, their scientific interest, and how they are incorporated into the overall observing schedule in terms of target selection and mission planning. What has been done so far and the preliminary results for a few ToO examples are then presented.