This report discusses basic methodology of life-cycle assessments and compares several publicly-available assessments of life-cycle emissions data for Canadian oil sands crudes against each other and against those of other global reference crudes. It also includes a survey of the scientific literature and the findings of the State Department's Keystone XL Project Envoronmental Impact Statement, and concludes with a discussion of tools for policymakers who are interested in using the assessments to investigate the potential impacts of U.S. energy policy choices onthe environment.