We study a dynamic model of labor migration in which moving costs decrease with the number of migrants already settled in the destination. This assumption is supported by sociological studies of migrant networks. With endogenous moving costs migration occurs gradually over time. Once it starts, it develops momentum, and migratory flows may increase even as wage differentials narrow. In addition, migration tends to follow geographical channels, and low-moving-cost individuals migrate first. These patterns are consistent with historical evidence from the Great Black Migration of 1915-1960, much of which cannot be reconciled with existing migration models.