What Is A Visa?
A U.S. non-immigrant visa is issued in an individual's passport by a U.S. consul in a U.S. embassy or consulate outside the U.S. A visa is used solely for the purpose of entry and/or reentry to the U.S. A visa does not determine how long an individual may remain in the U.S. Therefore, an individual may stay beyond the expiration date of a visa as long as the visa document (Form I-20, Form DS-2019, or Form I-797) is valid. Most visas are for multiple entries. Please note that it is not possible to apply for or to renew a visa inside the U.S.
Canadian citizens are not required to obtain U.S. visas in their passports for entry into the U.S. Instead of a visa, Canadian citizens are currently admitted to the U.S. in a particular visa status by presenting their valid Canadian passports and the appropriate visa document for their visa status e.g. Form I-20, Form DS-2019 etc.
International students and scholars at Harvard should use the appropriate visa related to their current activities at Harvard (a student visa or a research scholar visa). Use of a "visitor" or "tourist" visa may have serious consequences when used inappropriately. Once the proper visa expires it will be necessary to apply for a new visa in a U.S. embassy or consulate when traveling outside the U.S.