Depending on how a person is seeking to enter the US, different documents are required before the INS will admit the person. A person seeking to enter as an immigrant will not be admitted unless they have a valid unexpired immigrant visa and a valid unexpired passport. Those seeking to enter as nonimmigrants must have a valid nonimmigrant visa or border crossing card, and a passport valid for at least six months past the date of entry. For many nonimmigrants, the documentary requirements can be waived.
The most obvious examples of people seeking to enter the US without valid documentation are undocumented border crossers. They are considered to be immigrants not in possession of a valid immigrant visa. Other circumstances when this rule applies are when returning residents have not complied with documentary requirements, such as a person in the middle of adjustment of status who seeks to reenter without an advance parole document, or a lawful permanent resident who has not obtained a reentry permit, if one is required.
If the documents that a person is using in seeking entry to the US are not valid or are not properly issued, the person is inadmissible. Fraudulent documents, of course, fall under this provision, as do documents that were obtained by fraud. A person seeking entry on a nonimmigrant visa that an INS inspector determines in fact wants to remain permanently in the US is inadmissible as an immigrant not in possession of a valid nonimmigrant visa. Also, if a person is in possession of a immigrant visa and it is determined that they were not in fact entitled to it, they are inadmissible even if the visa is otherwise valid.
There are a number of waivers of and exceptions to the documentary requirements, especially for nonimmigrants. The following groups of nonimmigrants are excluded from the documentary requirements: uniformed members of the US armed forces seeking entry on official business, Native Americans born in Canada, people entering the US from Guam, Puerto Rico and the US Virgin Islands, certain NATO personnel, and certain employees of the International Boundary and Water Commission between the US and Mexico. Any national of Canada and Mexican nationals who have a valid border-crossing card are not required to have a visa. Perhaps the largest waiver for nonimmigrants is the Visa Waiver Program, under which citizens of 29 different countries can enter the US for up to 90 days without a visa.
There are also a number of situations in which waivers can be granted on an individual basis. These waivers can be granted only by the District Director of the INS office with jurisdiction over the port where entry is sought, and are primarily granted in emergencies.