An A-Number is a 7, 8 or 9-digit number which is given to noncitizens when they enter the country. Each number is unique and identifies only one individual. The A-Numbers are given out by the Department of Homeland Security, and are also known as USCIS Numbers.

The A-Numbers are commonly found on permanent resident cards (Green Cards) underneath the USCIS Number. Other documents that bear an A-Number include an Employment Authorization Card (work permit) and a notice of action.

In order to acquire the A-Number, one has to apply for immigrant status. This requires the alien to go through a rigorous eligibility process before being assigned the designated immigration status. Most immigrants are sponsored by close family members. In the case of coming to work in the United States, it would be the role of the employer to act as the petitioner. Other categories of people who can gain permanent resident status and get an A-Number include people seeking asylum and refugees who have been accepted into the country after screening.

The A-file, which contains the A-Number, is usually created when one has gone through the entire application and processing process, and has been officially accepted as an immigrant into the United States. Towards the end of this process, immigrants usually have an appointment in the US embassy in their homeland for a visa review which includes an interview. They are also given documents containing their unique A-Number as well as their Department of State Case ID.

The A-Number is necessary when applying for housing and utilities, opening bank accounts, applying for jobs and more. It is, therefore, a vital number that one should not lose or forget.