A household's decision to send migrants is based on information the household has on the expected returns and the costs of migration. Information on migration flows from both family migrant networks and community migrant networks. Direct assistance - in the form of money, housing, transportation, and food - is often provided to migrants by these networks, thus reducing the costs of migration. Using data from a national survey of rural Mexican households, we show the importance of networks in both the decision to migrate and the level of migration. We find that community and family networks are substitutes in the production of information and assistance suggesting that, once migration is well established in a community, family networks become less important. In addition, the development of strong community networks erases the role of household characteristics in migration, allowing those initially least favored to also participate in migration. Results suggest that policies designed to reduce Mexico-U.S. migration should focus on regions where migrant networks are yet weakly developed since, once strong community networks become established, reducing migration would require much higher levels of public investment.