All nonimmigrants require a valid visa stamp in their passports before coming to the U.S. to work or study. Exceptions to this general rule include citizens of Canada or contiguous islands that are considered visa exempt.
In general, a valid visa stamp is also required for nonimmigrants returning to the U.S. after travel abroad. An exception to this general rule exists when a student travels only to Canada or Mexico (and certain contiguous territories) and complies with the automatic visa revalidation standards.
Visa stamps in passports are only for admission to the U.S. in a specific status, or visa "type". After admission, visa stamps do not have any substantive impact on one's status in the U.S. Therefore it is not necessary for nonimmigrants maintaining status in the U.S. to possess a valid visa stamp in their passport. An expired visa may only need to be renewed for re-entry into the U.S.
Visit http://usembassy.state.gov/ for a list of embassies, consulates and diplomatic missions. Each Web site will have specific visa application procedures for your area. Also visit http://travel.state.gov for information about visa wait times in your area. Note that visa wait time information is not always accurate. Visa application procedures, appointment times and visa issuance times are subject to change without notice.
Visa application appointments must be scheduled in advance. You should plan to schedule your visa application appointment at least one month in advance. In some instances, applications must be scheduled many months in advance. Please note that you should ensure that you schedule your visa application appointment for a date when you will have all required materials. Visa applications will often require a document from the U.S. such as an I-20, DS-2019 or I-797. You should ensure that you will have this document at the time of your application.
F-1 and J-1 applicants (among others) must demonstrate to the U.S. consulate that they do not plan to reside in the U.S. permanently. These applicants should be prepared to provide the consulate with ample documentation that they plan to return to their home country. F-1 and J-1 applicants must also pay the SEVIS fee (SEVIS is the Student and Exchange Visitor Information System) prior to their applications.