Optical steppers have opened their new era beyond 1 Am barrier. In order to achieve submicron lithography, however, steppers have been obliged to solve many problems that were not noticed before. For example, reduction lenses that were recognized as stable components have now become dynamic and intelligent system. There are three basic parameters in the projection optics---- resolution, depth of focus and distortion. As for resolution, the uniformity of the image quality over the whole field is required. The SEM image of 0.8 Am and its uniformity are discussed and some simulation results are shown. Depth of focus is the second point and is recognized as one of the most critical parameters in optical lithography. In order to achieve large depth of focus, new optical auto-focus system is introduced. This system has two features. One is the grazing angle of incidence and the other is the multiple wavelength effect. The third point is distortion, which is the final key for overlay accuracy. The stability of distortion due. to barometric compensation and the machine to machine mixture problems will be discussed in detail.