A review is presented on focus effects in optical lithography. Alternative definitions of resolution and depth of focus are given based on an understanding of the interactions of the aerial image with the photoresist process. This interaction points to various aspects of the aerial image that are important from a lithographic point of view, especially the aerial image log-slope. The effects of numerical aperture, wavelength, feature size, and feature type can also be characterized using the log-slope defocus curve, thereby permitting objective comparisons of different lithographic tools. The impact of the photoresist on the response of the process to changes in focus is described as two major effects. First, improving the photoresist results in improved exposure latitude. This in turn allows the image to be further degraded by focus errors and still give acceptable results. Second, submicrometer optical lithography usually results in asymmetrical focus behavior because of the defocusing of the aerial image as it propagates through the photoresist. Finally, several methods for depth-of-focus improvement are discussed and their relative merits and drawbacks are reviewed.