The Alien Registration Act of 1940 required all persons who were not citizens or nationals of the United States and were living within U.S. borders go to the local post office and register their alien status with the government. The registration process included filling out a questionnaire and having fingerprints taken. Certain exclusions applied for diplomats, employees of foreign governments, and children under the age of 14.

The Alien Registration Form (Form AR-2) contained 15 questions. These included questions about when and where the subject was born; when and where they entered the United States; and a physical description. There were also questions about employment, organization memberships, prior military service and attempts to obtain naturalization. As the Immigration and Naturalization Service (INS) received the forms, it assigned an Alien Registration Number (for example, A1234567). The INS mailed an Alien Registration Receipt Card with this number to each registrant as proof of alien status.

In addition to the link to the searchable database, the following links also provide further information about registration and the documents.
World War II Alien Registration Files: Search the Database
      World War II Alien Registration Files: Background
      World War II Alien Registration Files: General Description
      World War II Alien Registration Files: Form AR-2 Described