The proximity effects are becoming a significant consideration to the linewidth control in sub-micron optical lithography, as the lithographic tools are being operated closer to the resolution limit. There are various forms of proximity effects. In this paper, we will attempt to classify the observed proximity effects in optical lithography and analyze their potential impact to device fabrication. The results of SAMPLE simulation and experimental characterization will be presented. Our conclusion from this study is that a method of linewidth compensation must be developed in order to achieve optimum device density and yield, using sub-micron optical lithography.