This joint IPIECA-API document is designed to provide a summary of technical considerations that are important for understanding and calculating greenhouse gas (GHG) emission inventory uncertainty. It includes technical background and specific calculation methods to determine uncertainties with targeted measurements and emission factors, and to determine how to aggregate these individual terms to derive uncertainty ranges (as a pre-designated probability level) for entire GHG inventories at any given level. This document builds on a 2007 IPIECA/API workshop where the need to develop an understanding of the importance of the key factors that contribute to uncertainty was established, a pilot version was subsequently published in 2009 to allow for a broad review and implementation of guidance from various stakeholders. The comments received, lessons learned and new developments that have occurred in the area since then are incorporated in this document. Additional Information This document is a companion to the API Compendium of Greenhouse Gas Emissions Methodologies for the Oil and Natural Gas Industry (API, 2009).