Dynamically changing the aggressiveness of optimization of virtual machines on physical hosts allows more efficient and varied optimization. An aggressiveness policy mechanism periodically applies system conditions to the aggressiveness policies to create aggressiveness settings that are provided to an optimizer. The optimizer then uses the aggressiveness settings to dynamically adjust the aggressiveness of placement of virtual machines according to the aggressiveness settings and consistent with other optimization policies. The aggressiveness policy mechanism may allow a system administrator to create and/or select aggressiveness policies.