A system for reactive control of the thermal infrared (TIR) signature is described. The system is primarily developed for ground vehicles, but can easily be adapted to other platforms. The main parts of the system are a sensor package for background and vehicle signature awareness, actuators for heating/cooling of the vehicle surface and control algorithms to connect the sensor signals with the actuator control. Some of the more important aspects of the development process are discussed, including physical modeling and simulation. Physical modeling has been a useful tool, and the models are also included as part of the control algorithms. Reactive signature control should be regarded as a component in a Defensive Aids Suite (DAS). The crew is provided with the current signature status as well as high-contrast warnings. The signature control system can also be considered as a countermeasure, since it reduces the probability for detection as well as the hit probability for heat-seeking missiles. The system will soon be implemented and tested in a ground technology demonstrator.