This invention concerns an additive manufacturing apparatus for building objects by layerwise consolidation of material. The apparatus comprises a build chamber 101 containing a working area, a plurality of high energy beams 133, 233 for consolidating material deposited in the working area in layers and an optical unit 135 for controlling transmission of the high energy beams onto material in the working area. The optical unit 135 comprises a plurality of independently controllable optical elements 141, 241 each optical element 141, 241 for controlling transmission of at least one of the high energy beams onto the material in the working area, the optical unit 135 movable in the build chamber 101.