The A4 project will introduce the concept of adaptivity into mass storage systems. The central idea is to vary the quality of service (QoS) provided to the user, depending on the available power budget. You could for instance reduce the quality of the video you are watching if you wish to conserve battery power. Current mass storage devices, like Flash memory and hard-disk, will be considered. They are however not very adaptive. Therefore we will also look at new systems based on probe storage.