Optical lithography is moving into an era of refinement where lenses are continuously improving but must be superb to utilize the resolution enabled by new complex reticles. Phase Measuring Interferometry used during lens fabrication is not available to the lithographer who must rely on ingenuity in reticle design to form measurable, aberration sensitive images in photoresist. This ingenuity, as reported in the papers that follow, is enabling lithographers to measure lens aberrations on their tools with a precision nearly that of a PMI. The principles behind these photoresist methods are reviewed in this paper to help enable critical review of those that follow.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.