Naturalization is the process by which a person born outside the United States becomes a U.S. citizen. In a nation of immigrants, it is the most important step that an immigrant can take to establish ties with the United States. This remains as true for the Hispanic immigrants of today as for the European immigrants of past decades. Some benefits of citizenship are quite tangible. For example, U.S. citizens have greater access to immigrant visas for their family members than do permanent residents. They are the only group who can obtain visas for their spouses and unmarried minor children almost immediately, and who have the right to petition for their married children, parents and siblings. Permanent residents may only petition for their spouses and unmarried children and must do so through the preference system. The preference system is subject to numerical limits, which have caused huge backlogs in recent years.