The inertia switch device (1) comprises an inertia mass (6) which is movable from a rest position when the device is subjected to an applied acceleration or deceleration in a horizontal plane, a first electrical contact (5) movable between a first stable position in which it engages a second electrical contact (3) and a second stable position in which it no longer engages the second electrical contact, and support means (4) having a bifurcated portion (24) for carrying a bifurcated portion of the first electrical contact (5). An operating means (27) is struck by the inertia mass (6), when the applied acceleration or deceleration exceeds a threshold value and moves the first electrical contact (5) from the first to the second stable position. The operating means (27) has a first limb (31) which is struck by the inertia mass and a second limb (28) with a bifurcated portion which is pivotally mounted on the support means. A resilient biassing means (34) lies within the bifurcated portions and connects the first electrical contact (5) and the second limb (28) so as to bias the first electrical contact (5) towards the first or second stable position. A resetting means has an operating member (35) movable to operate the resilient biassing means (34) and thereby return the first electrical contact (5) to its original stable position.