Active safety and collision avoidance (CA) is a growing field within the automotive industry. The aim of CA systems is to prevent or mitigate collisions by active interventions i.e. warning, braking and steering. For many reasons, such as driver acceptance of the system and legal requirement that the system itself must not cause hazards, the decision making is a crucial part of the system. This paper presents a method for risk estimation on which to base the decision making. We will show how on can form criterions for decision making in terms of probability of collision. This criterion handles the noisy sensor data and process noise (driver behavior) in a natural way, using modern tracking theory. The method is illustrated by simulation results as well as test result from a prototype vehicle. Simulations and tests are examples of a system which performs autonomous braking actuation at imminent collision.