A lithographic apparatus comprises a substrate table that supports a substrate having alignment marks on a surface thereof. The apparatus further comprises a frame moveable relative to the substrate to provide for a scanning or stepping mode of operation. An array of projection systems is disposed across the frame for projecting respective patterned beams onto a target portion of the substrate. A plurality of alignment mark detectors are attached to the frame and are moveable with respect to the frame using respective linear drive mechanisms. A position sensor is associated with each alignment mark detector for determining the position of the detector relative to the frame. A control system is responsible for both initial positioning of the detectors above alignment mark patterns on the substrate, and for dynamic alignment of the frame and substrate during a lithographic process.