Practical resolution limits of submicrometer patterning by optical lithography are estimated by computer simulation. As a result, the optimum numerical aperture NA that gives the highest resolution is determined. Assuming that the permitted defocus value is ± 1 µm, a litho-graphic resolution of about 0.7 µm is obtained with 0.5 NA and near-UV exposure. A 0.5 µm resolution is obtained with 0.35 NA and deep-UV exposure. It is suggested that resolution of less than 0.4 µm will be realized by the use of resist system technologies, optical exposure tools that optimize NA as well as exposure wavelength, and precise focusing control of less than ± 0.5 µm. Furthermore, it is predicted that the goal of 0.4 to 0.5 µm resolution will be achieved industrially before the end of the 1980s by the use of optical lithography.