The invention relates to a method of enabling multiple operating systems to run concurrently on the same computer, the method comprising: scheduling a plurality of tasks for execution by at least first and second operating systems, wherein each task has one of a plurality of priorities; setting the priority of each operating system in accordance with the priority of the next task scheduled for execution by the respective operating system; and providing a common program arranged to compare the priorities of all operating systems and to pass control to the operating system having the highest priority. Accordingly, the invention resides in the idea that different operating systems can be run more efficiently on a single CPU by changing the priority of each operating system over time. In other words, each operating system has a flexible priority.