Unlike most countries, China regulates internal migration. Public benefits, access to good quality housing, schools, health care, and attractive employment opportunities are available only to those who have local registration (Hukou). Coincident with the deepening of economic reforms, Hukou has gradually been relaxed since the 1980s, helping to explain an extraordinary surge of migration within China. In this study of interprovincial Chinese migration, we address two questions. First, what is a sensible way of incorporating Hukou into theoretical and empirical models of internal migration? Second, to what extent has Hukou influenced the scale and structure of migration? We incorporate two alternative measures of Hukou into a modified gravity model – the unregistered migrant's: (i) perceived probability of securing Hukou; and (ii) perceived probability of securing employment opportunities available only to those with Hukou. In contrast to previous studies, our model includes a much wider variety of control especially important for the Chinese case. Analyzing the relationship between Hukou and migration using census data for 1985-90, 1995-2000 and 2000-05, we find that migration is very sensitive to Hukou, with the greatest sensitivity occurring during the middle period.