All nonimmigrants require a valid visa stamp in their passports before coming to the U.S. to work or study. An exception to this general rule exists for citizens of Canada (and Bermuda under limited circumstances) who are visa exempt, thereby not requiring a visa stamp.
In general, a valid visa stamp is also required for nonimmigrants returning to the U.S. after travel abroad. An exception to this rule exists when traveling only to Canada or Mexico (and in some circumstances, certain contiguous territories) and complying with the automatic visa revalidation standards.
Importantly, visa stamps in passports are only for admission to the U.S. in a specific status. After admission visa stamps do not have any substantive impact on one's legal status in the U.S. Accordingly, it is unnecessary for nonimmigrants maintaining status in the U.S. to possess a valid visa stamp in their passport while residing in the U.S. An expired visa should be renewed following a departure from the U.S. with the intent to return in the corresponding status.
Visit http://usembassy.state.gov/ for a list of embassies, consulates and diplomatic missions. Each website will have specific visa application procedures for your area. Also visit travel.state.gov/visa/temp/wait/tempvisitors_wait.php for information about visa wait times in your area. Note that the visa wait times 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 allow at least one month to schedule your visa application appointment. 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 requisite 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 prior to their applications.