A control system is disclosed for operating an engine valve. The system comprises two cams arranged on a common camshaft but rotatable relative to one another. A summation lever has cam followers in contact respective cams. The summation lever is movable in proportion to the instantaneous sum of the lifts of the respective cams. Rockers are provided for opening the engine valve in dependence upon the movement of the summation lever. By varying the phase of the cams relative to one another, it is possible to vary the valve timing, valve lift and/or valve event duration.