The BESS model is a computer program for assessing natural resource requirements,  energy balance and efficiency, and greenhouse gas emissions of a given corn-to-ethanol system. Such a system can be composed of up to four components: corn production, ethanol biorefinery, cattle feedlot using distillers grains from the ethanol biorefinery (optional), and biogas generation from cattle manure and utilized by the ethanol biorefinery (optional). The assessment is at the component level and for the entire system. The program provides numerical results as well as graphic plots. It also has specific graphic plots for scenario comparisons of material inputs, energy balance and efficiency, and net greenhouse gas emissions and credit, thus making the program ideal for “what-if” analysis across different scenarios. The program also generates a summary report that includes simulation results, input settings, and internal model parameters. All internal model parameters are transparent and can be modified by the user. However, when the value of an internal model parameter is modified by the user, it will be highlighted in the summary report with its current value along with its original default value, making the program highly suitable for certification purposes. The programs comes with a user guide (in pdf format) that provides operation instructions, as well as detailed documentation on default model parameters and input settings, and assumptions. The program has a full graphic user interface and is highly intuitive to use. Numerical outputs can be directly loaded to a spreadsheet program (e.g., MS Excel) for further processing and analysis, while graphic outputs can be saved and embedded into other documents. The BESS model has a dedicated website at www.bess.unl.edu that provides more information about background of the program project , its development and update. Moreover, the program is free for download at its website.