Abstract: Apparatus for adjusting the timing of a valve, particularly but not exclusively for use with a four-stroke internal combustion engine. The apparatus includes a cam, which engages a cam follower which is arranged to actuate a valve mechanism in response to rotation of the cam. The cam follower is moveable in translation relative to the axis of rotation of the cam to vary the extent of actuation of the valve in response to rotation of the cam. The cam follower is shaped so that for a part of a cycle of rotation of the cam, the line of action of the force from the cam to the follower forces the follower to move in translation. This movement is greater at low cam rotational speeds than at high cam rotational speeds and, due to the profile of the surfaces of the follower, the translational movement provides varying extents of valve actuation in dependence upon cam rotational speeds.