1. An inspection apparatus for measuring a property of a lithographic process using a composite target structure including a plurality of component structures that have been formed by the lithographic process on a substrate, the apparatus comprising: a support for the substrate having the composite target structure formed thereon; an optical system for illuminating the composite target structure under predetermined illumination conditions and for forming and detecting an image of the composite target structure using a predetermined portion of radiation diffracted by the component target structures under the illumination conditions; a processor arranged to identify one or more regions of interest in the detected image, the or each region of interest corresponding to a specific one of the component target structures and to process pixel values within the region of interest to obtain a measurement of the property of the component structure, wherein the processor is arranged to identify the regions of interest such that their boundaries fall within image regions corresponding to separation zones between the component structures within the composite target structure.