Depending on your legal status with the United States, you may be required to show different documents at the port-of-entry.

• U.S. Citizen
  In most cases, a U.S. passport is required for international travel. There are some exceptions that are being phased out gradually.
  
  
• Lawful Permanent Resident of the United States
  One of the following documents:
  • Permanent Resident Card ("Green Card" or Form I-551)
    
    
  • Reentry Permit (if out of the U.S. for more than one year)
    
    
  • Returning Resident Visa (if out of the U.S. for 2 years or more)
    
    
• Persons on Immigrant Visa
  • Passport
    
    
  • Entire package given by the U.S. Embassy/Consulate, including immigrant visa.
    
    
  More Details
  
  
• Nonimmigrants
  • Passport
    
    
  • Category specific
    • Visa Waiver Program
      No visa is required, as long as you are applying to enter the U.S. as a visitor for business or pleasure (B-1 or B-2), if you are staying for no more than 90 days, and if not admissible.
      
      
    • Canadians:
      Canadians generally don't require a visa. There are a few exceptions. Canadians must prove their identity and citizenship (birth certificate, citizenship certificate or passport) to enter the U.S. If you don't have such documents because they are lost or stolen, the burden of proof is on you to prove that you are Canadian citizen. Copies of correspondence requesting a replacement of documents, etc. might be accepted, but it is up to the CBP officer to determine whether or not such paperwork meets that burden of proof.
      They can visit up to 6 months.
      
      Canadians coming as a Treaty Trader (E classification) are required to have a visa to enter the U.S.
      
    • Mexicans:
      Form DSP-150, B-1/B-2 Visa and Border Crossing Card, containing a machine-readable biometric identifier, issued by the Department of State and is applying for admission as a temporary visitor for business or pleasure from contiguous territory. In such as passport and a visa are not required. However, if the traveler is not coming from the Western Hemisphere, a passport is required.
      
      
    • Advance Parole, for whom the application to adjust the status to permanent residence (I-485) is pending and who temporarily traveled outside the U.S.
      
      Effective Nov 1, 2007, I-485 receipt is not required.
      
      Also, read EAD/AP vs. H/L status.
      
      
    • All others
      
      
      • Valid, unexpired visa stamp in the passport for the category under which you are applying to enter the U.S.
        
        
      • Category Specific
        • F-1 or M-1 Visa
          • Current Form I-20, endorsed by the Designated School Official (DSO) or RO for Travel. The endorsement will be made on page 3 of the SEVIS Form I-20.
            
            
          • Form I-901 fee payment receipt.
            
            
        • J-1 Visa
          • Form DS-2019 (filled out in blue ink). If studying at school, Form DS-2019 must be endorsed by the Designated School Official (DSO) or RO for Travel. The endorsement will be made on page 1 of the Form DS-2019.
            
            
          • Form I-901 fee payment receipt.
            
            
        • Prior Visitor Visa Extension
          • Original visitor visa extension approval notice.
            
            
        • H-1 Transfer
          • If you don't have a visa stamp corresponding with your current employer, H-1 visa status approval - notice of action.