During the last decades various low-power states have been implemented in processors. They can be used by the operating system to reduce the power consumption. The applied power saving mechanisms include load-dependent frequency and voltage scaling as well as the temporary deactivation of unused components. These techniques reduce the power consumption and thereby enable energy efficiency improvements if the system is not used to full capacity. However, an inappropriate usage of low-power states can significantly degrade the performance. The time required to re-establish full performance can be significant. Therefore, deep idle states are occasionally disabled, especially if applications have real-time requirements. In this paper, we describe how low-power states are implemented in current x86 processors. We then measure the wake-up latencies of various low-power states that occur when a processor core is reactivated. Finally, we compare our results to the vendor’s specifications that are exposed to the operating system.