Virtualization is a long established technology in the server world that has also gained new public interest due to its availability for Desktop PC platforms. The fact that it can provide strong isolation between applications and that this level of isolation can be achieved with only a very small layer of trusted code has also raised interest from the security related field of applications. The possibility to integrate multiple independent subsystems in a single physical machine could also prove beneficial for many safety-critical applications. This paper looks at the requirements for virtual machine monitors which result from supporting a mixture of hard real-time, soft real-time and best-effort applications.