This paper first discusses the advancement and challenges in the areas of developing Collision Avoidance Systems, or CAS. CAS have been on the market for a decade, and their development has been rapid. Starting with forward collision warning with brake support, targeting vehicles moving in the same direction in front of the car, CAS now cover pedestrians and cyclists in front of the car as well as vehicles standing still and even some situations of approaching vehicles in crossings. This development up to date is described and discussed according to the challenge areas of detection, decision strategy and intervention strategy.Next, the paper discusses assessment of system effects on driving safety. Numerous studies have tried to predict the effect of various CAS, and the real world effect of these systems has been shown to be significant. For example, for a standard equipped low-speed CAS, independent studies have found real world benefits in terms of reduction of the rate of rear-end frontal impacts as well as claim frequency rates.Based on these discussions, several conclusions are drawn. First, the discussion on technical development shows that many CAS are reaching maturity from a technical standpoint. Second, a clear conclusion from the effect studies is that a high market penetration is essential to achieve real-world safety. Standard equipped systems are likely to provide higher real-world benefits, simply through the exposure to traffic they provide.Together, these lead to a third conclusion which is that while continued technological development remains a core activity, in the future it will be just as important to develop new and improved strategies for pushing consumer take up of CAS. The full safety potential of CAS can only be realized through widespread presence in the vehicle fleet. Finally, if such strategies are successful and CAS fitment rates in the fleet indeed do increase, a shift in the focus of CAS development can be expected. As more and more drivers will need to successfully interact with CAS in critical situations, drivers' acceptance of the CAS becomes critical. Since drivers who have systems fitted as standard rather than option can be expected to show lower acceptance, it is quite possible that future CAS success may come to depend just as much on HMI design as it does on technological prowess.