The availability of affordable Micro Aerial Vehicles (MAVs) opens up a whole new field of civil applications. We present an Infrastructure for Compact Aerial Robots Under Supervision (ICARUS) that realizes a scalable low-cost testbed for research in the area of MAVs starting at about $100. It combines hardware and software for tracking and computer- based control of multiple quadrotors. In combination with the usage of lightweight miniature off-the-shelf quadrotors our system provides a testbed that virtually can be used anywhere without the need of elaborate safety measures. We give an overview of the entire system, provide some implementation details as well as an evaluation and depict different applications based on our infrastructure such as an Unmanned Ground Vehicle (UGV) which in cooperation with a MAV can be utilized in Search and Rescue (SAR) operations and a multi-user interaction scenario with several MAVs.