A seat-belt pretensioner arrangement includes a pretensioner configured to apply at least two tension levels to a seat-belt and a control system to control the pretensioner. A control system is configured to provide at least one vehicle slip function which is derived from at least one sensed parameter relating to the current or expected dynamic situation of the vehicle. The control system is configured to select the tension level to be applied to the seat-belt by the pretensioner in response to the value of the slip function provided by the control system.