Sanctuary cities are places in the US where local or state governments set limits on how they work with federal immigration officers. There is no official legal definition, and each city, county, or state can choose its own approach. In general, these policies focus on how information is shared, not on hiding people. Many sanctuary policies deal with immigration detainers. A detainer is a request from federal authorities asking local police to hold someone so immigration officers can take custody. Some jurisdictions choose not to hold people based only on these civil immigration requests. Sanctuary cities do not shelter undocumented immigrants or block federal enforcement. Local police still enforce state and local laws and arrest people accused of crimes, regardless of immigration status. The idea of sanctuary is not new. In 1971, Berkeley became a sanctuary city by offering a place to Navy members who opposed the Vietnam War there.