Data centers hosting internet-scale services consume megawatts of power. Mainly for cost reasons but also to appease environmental concerns, data center ope rators are interested to reduce their use of energy. This thesis investigates if and how hardware virtualization helps to improve the energy efficiency of modern cloud data centers. Our main motivation is to power off unu First, a simulation-driven analysis to quantify the benefits of known reservation times in infrastrulocated to increase the probability to power down unused physical hosts. Second, we propose and protdemand cloud services. Idle virtual machines are suspended to free resources and as a first step to power off the physical server. T a novel block-level data synchronization tool enables fast and efficient state replication. Frequent state synchronization is necessary to prevent data unavailabi powering down a server disables access to the locally attached disks and any data stored on them. The techniques effectively reduce the overall number of required servers either through optimized scheduling or by suspending idle virtual machines. Fewer live servers as the unused servers must no longer be powered.