Method for estimating a vehicle's, or, in a vehicle trailer combination having a tractor vehicle and a towed trailer, a tractor vehicle's sideslip angle, the method comprising the steps of: a) Gaining the following vehicle parameters from the vehicle: Axle distance, track width, rolling tire radiuses, and steering wheel angle, b) Measuring signals for the longitudinal velocity, the steering wheel angle and the yaw rate of the vehicle, where c) the method for estimating operates with two iterations in each processing step, where a processing step is done by a vehicle's onboard integrated processing unit in real time, where c1) a first iteration estimates the vehicle sideslip angle's next value on the basis of the gained vehicle parameters and the measured signals, c2) a second iteration estimates a mass specific cornering stiffness parameter on the basis of the gained vehicle parameters and the measured signals, where c3) between the two iterations, a regulation method is inserted, which is based on constraints, which are originated from physical laws such as continuity or maximum possible adhesion coefficient, where a first regulation step calculates threshold values for the vehicle sideslip angle that are based on kinematical conditions, and a following regulation step investigates that the continuity of the vehicle sideslip angle can be held or not with the calculated maximum and minimum vehicle sideslip angles, and if yes then saturates the first iteration's result. a) Gaining the following vehicle parameters from the vehicle: Axle distance, track width, rolling tire radiuses, and steering wheel angle, b) Measuring signals for the longitudinal velocity, the steering wheel angle and the yaw rate of the vehicle, where c) the method for estimating operates with two iterations in each processing step, where a processing step is done by a vehicle's onboard integrated processing unit in real time, where c1) a first iteration estimates the vehicle sideslip angle's next value on the basis of the gained vehicle parameters and the measured signals, c2) a second iteration estimates a mass specific cornering stiffness parameter on the basis of the gained vehicle parameters and the measured signals, where c3) between the two iterations, a regulation method is inserted, which is based on constraints, which are originated from physical laws such as continuity or maximum possible adhesion coefficient, where a first regulation step calculates threshold values for the vehicle sideslip angle that are based on kinematical conditions, and a following regulation step investigates that the continuity of the vehicle sideslip angle can be held or not with the calculated maximum and minimum vehicle sideslip angles, and if yes then saturates the first iteration's result. c1) a first iteration estimates the vehicle sideslip angle's next value on the basis of the gained vehicle parameters and the measured signals, c2) a second iteration estimates a mass specific cornering stiffness parameter on the basis of the gained vehicle parameters and the measured signals, where c3) between the two iterations, a regulation method is inserted, which is based on constraints, which are originated from physical laws such as continuity or maximum possible adhesion coefficient, where a first regulation step calculates threshold values for the vehicle sideslip angle that are based on kinematical conditions, and a following regulation step investigates that the continuity of the vehicle sideslip angle can be held or not with the calculated maximum and minimum vehicle sideslip angles, and if yes then saturates the first iteration's result.