A seat-belt pretensioner arrangement comprises a pretensioner (7) and a control system (3-6) to control the tension applied to a seat-belt by the pretensioner (7). The control system (3-6) controls the pretensioner (7) in response to at least the rate of change of steering angle (δ′). A first processor (4) provides an output indicating a desired level of force to be applied to the seat-belt determined from the steering angle and the velocity of the vehicle. A second processor (5) provides a corresponding output indicative of a desired force level, determined from the rate of change of steering angle (δ′) and the velocity of the vehicle (vx). A selector (6) selects the highest of the two force levels, and provides an appropriate signal to the pretensioner (7).