This thesis describes the design and implementation of a real-time criminal tracking system. A criminal under tracking is asked to wear a low energy Bluetooth device and carry a smartphone. The Bluetooth device is secure on his body (e.g., hand or foot) and communicates with the smartphone, which communicates with a central server through cellular networks. The smartphone monitors the status of the Bluetooth device and reports to the server in real time when the status changes (e.g., connection lost or device being taken off). Moreover, it monitors the criminal's movement and reports to the server whenever the criminal moves into an alert zone, a geographic region where the law enforcement wants the criminal's movement to be tracked. Compared to the existing tracking approaches, our system has the following desired features. 1) Scalable. Instead of having a criminal to report its location all the time, the system allows one to configure where and when the criminal needs to be tracked, thus minimizing both mobile communication cost and server processing cost. 2) Low-cost. The system uses only off-shelf components (e.g., Bluetooth device and smartphone) which communicate through regular wireless networks. 3) Secure. The communication between a Bluetooth device and the corresponding smartphone is authenticated through One Time Password.