This guidance document describes the procedures, data evaluation criteria and associated tools and data management systems that the U.S. Environmental Protection Agency (EPA) recommends for developing air pollutant emissions factors for stationary emissions units or processes. This document supersedes the previous EPA guidance document for emissions factor development (Procedures for Preparing Emission Factor Documents (EPA-454/R-95-015, November 1997)). This document presents an introduction to emissions factors and provides the historical background for how and why the EPA has developed emissions factors for stationary emissions units or processes. This document also describes the approach and procedures recommended by the EPA for developing new or revising existing emissions factors. This document provides an overview of the EPAs WebFIRE an online data storage and emissions factor retrieval and development tool. Also discussed is the EPAs Electronic Reporting Tool (ERT) which is a Microsoft Access application that facilitates the development and documentation of emissions test reports. In addition, the procedures that must be followed by individuals and entities submitting emissions data and related process data to WebFIRE are also presented in this document. Finally, this document provides an overview of the data review and public participation process that the EPA follows when developing new or revised emissions factors.