The United States (US) Federal Aviation Administration (FAA) Office of Environment and Energy (AEE) has developed the System for assessing Aviation's Global Emissions (SAGE) with support from the Volpe National Transportation Systems Center (Volpe), the Massachusetts Institute of Technology (MIT) and the Logistics Management Institute (LMI). Currently at Version 1.5, SAGE is a high fidelity computer model used to predict aircraft fuel burn and emissions for all commercial (civil) flights globally in a given year. As part of SAGE development, this document has been prepared to provide detailed specifications of the model's methods, data, and assumptions. The information is provided within the context of using SAGE to develop global inventories of fuel burn and emissions.