This report discusses methods of measuring unauthorized migration to the United States. The “residual method” involves comparing an analytic estimate of the legal foreign-born population with a survey-based measure of the total foreign-born population. The difference between the two population figures is a measure of the unauthorized migrant population in the survey; it can then be corrected for omissions to provide a measure of the total unauthorized population. The report includes a detailed description of the residual methods and the underlying data and assumptions as it has been applied to recent data from the Current Population Survey (CPS) and decennial censuses. The paper presents new results of estimates derived from the march 2006 CPS which show that the unauthorized population in the U.S. has reached 11.5 million; of these, 6.5 million or 57% are from Mexico. The report also presents derived data on a range of social and economic characteristics of the unauthorized population developed with an extension of the residual estimates. Finally, historical data on trends in unauthorized migration and several alternative estimation methods are presented and discussed.