Generally, a citizen of a foreign country who wishes to enter the United States must first obtain a visa, either a nonimmigrant visa for temporary stay, or an immigrant visa for permanent residence. Temporary worker visas are for persons who want to enter the United States for employment lasting a fixed period of time, and are not considered permanent or indefinite. Each of these visas requires the prospective employer to first file a petition with U.S. Citizenship and Immigration Services (USCIS). An approved petition is required to apply for a work visa.