Virtualization techniques are getting more popular. It allows running multiple virtual servers on a single physical machine. But in case of a hardware outage, tens of virtual servers are down. This paper published by Thomas Krenn.AG gives some insight how to prevent this situation. A problem of virtualization discusses why it is necessary to think about clustering in virtualized environments. Then, Types of virtualization explains characteristics of different virtualization techniques and shows some examples of virtualization. HA cluster configurations in virtual environments looks into different kinds of possible cluster configurations, their respective advantages and drawbacks, and possible pitfalls.