Machine generated contents note: 1.Introduction -- 2.The Process of Unauthorized Crossing at the U.S.-Mexico Border -- 3.Migration-Relevant Surveys in the United States and Mexico: Background -- 4.Migration-Relevant Surveys in the United States and Mexico: Usefulness and Limitations -- 5.Administrative Data on Undocumented Migration Across U.S. Borders -- 6.Model-Based Approaches to Estimating Migration Flows -- References -- Appendixes -- A.Survey Questions About Migration and Border Crossing -- B.Review of Capture-Recapture Ideas for Measuring the Flow of Unauthorized Crossings at the U.S.-Mexico Border.