A method and apparatus for inspecting a photolithography mask for defects is provided. The inspection method comprises providing a defect area image to an image simulator wherein the defect area image is an image of a portion of a photolithography mask, and providing a set of lithography parameters as a second input to the image simulator. The defect area image may be provided by an inspection tool which scans the photolithography mask for defects using a high resolution microscope and captures images of areas of the mask around identified potential defects. The image simulator generates a first simulated image in response to the defect area image and the set of lithography parameters. The first simulated image is a simulation of an image which would be printed on a wafer if the wafer were to be exposed to an illumination source directed through the portion of the mask. The method may also include providing a second simulated image which is a simulation of the wafer print of the portion of the design mask which corresponds to the portion represented by the defect area image. The method also provides for the comparison of the first and second simulated images in order to determine the printability of any identified potential defects on the photolithography mask. A method of determining the process window effect of any identified potential defects is also provided for.