1. A lithographic method comprising: patterning a beam of radiation with a patterning device, the patterning device comprising at least two image patterning portions and at least two metrology mark patterning portions; projecting at least two image portions of the patterned beam of radiation sequentially onto target portions of a substrate such that said projected image portions are substantially adjacent to each other on the substrate and collectively form a composite image on the substrate; projecting a metrology mark onto the substrate outside of the area of the composite image at the same time as projecting each of at least two of the image portions; and measuring the alignment of said metrology marks to determine the relative positions of the at least two image portions.