The U.S. economy has long relied on immigrant workers, many of them unauthorized, yet estimates of the inflow of unauthorized workers and the determinants of that inflow are hard to come by. This paper provides estimates of the number of newly arriving unauthorized workers from Mexico, the principal source of unauthorized immigrants to the United States, and examines how the inflow is related to U.S. and Mexico economic conditions. Our estimates suggest that annual inflows of unauthorized workers averaged about 170,000 during 1996-2014 but were much higher before the economic downturn that began in 2007. Labor market conditions in the U.S. and Mexico play key roles in this migrant flow. The models estimated here predict that annual unauthorized inflows from Mexico will be about 100,000 in the future if recent economic conditions persist, and higher if the U.S. economy booms or the Mexican economy weakens.