These are the simplest and one of the earliest form of control systems for robotics. Some Toys are a common example of these systems. They have a very fast response to the sensed information. However, the drawback of reactive systems is that they have to be very simple. The reactions should be simple and quick enough for the system to work in real-time.
These systems have negligible amount of intelligence, but a simple and well-defined behavior. This forms the basis of a Behavior controlled architecture or the Subsumption Architecture ( w:Subsumption architecture ). However, it's mentioned separately from the Subsumption Architecture as it can be used independently in its simplest form to make robots with low intelligence, but high real-time requirements.