One objective of this paper is to illuminate fuel economy and emission improvement mechanisms of variable cam timing systems and analyze their effects on engine control system design. By retarding or advancing the cam phase one can vary the engine volumetric efficiency, as well as the amount of exhaust gas that dilutes the air charge. Combining these effects with intake manifold and engine speed dynamics leads to a complex behavior of engine air-charge and torque that requires special handling by the engine control system. This paper reviews control algorithms for VCT engines that have been reported in the literature.