An engine is described having two camshafts 14, 16 each of which carries two groups of cams and comprises an inner shaft coupled for rotation with a first group of cams and an outer tube rotatably supported by the inner shaft and coupled for rotation with the second group of cams. A phaser 12 is provided to enable the phase of at least one of the two groups of cams on one of the SAP camshafts 14, 16 to be varied with reference to the phase of the engine crankshaft. Drive links in the form of meshing gear wheels, drive chains or belts, couple the two corresponding groups of cams on the respective camshafts for rotation in unison with one another.