Surface impoundments are used to treat, store, and dispose of wastewater generated by facilities in many different industries. Because surface impoundments are normally open to the atmosphere, the potential for air emissions of volatile organic compounds (VOC's) and toxic air pollutants exists. As such, State and local air pollution control agencies need a methodology to estimate the air emissions from surface impoundments. The Surface Impoundment Modeling System (SIMS) is a personal computer based program designed to estimate the air emissions from surface impoundments. The emission estimates are based on mass transfer models developed by the Emissions Standards Division (ESD) of the Environmental Protection Agency (EPA) during the evaluation of hazardous waste treatment, storage, and disposal facilities (TSDF's). SIMS allows the user to specify all the required inputs to these emission models when the information is available, or when only limited information is avaialble, provides default values for most of the model inputs. The manual presents a complete reference for all of the features and commands in SIMS. Another document entitled, Background Document for the SIMS, PB91-156729, discusses the emission models, surface impoundment design and operation, default parameter development, and the emission estimation procedure.