Many travelers to the United States do not need visas under the visa waiver program. To qualify for visa-free entry, travelers must be a citizen of one of the following countries:

Andorra, Australia , Austria, Belgium, Brunei, Denmark, Finland, France, Germany, Iceland, Ireland, Italy, Japan, Liechtenstein, Luxembourg, Monaco, New Zealand, Norway, Portugal, San Marino, Singapore, Slovenia, Spain, Sweden, Switzerland, the Netherlands, and the United Kingdom.
Effective 21 February 2002, Argentina is no longer included in the visa waiver program.

In addition to be being a citizen of one of the above countries, the travelers must be:

• traveling on an unexpired national or EC passport
• traveling for business or pleasure only (not government business)
• staying in the US for 90 days or less.

if entering the US by air or sea:

• holding a return or onward ticket
• entering the US aboard an air or sea carrier that has signed an agreement with the US Immigration and Naturalization Service to participate in the visa-waiver program; and in possession of a completed form I-94W, obtainable from airline or shipping companies.
  
  Or, if entering the US by land from Canada or Mexico:
• in possession of a completed form I-94W, issued by the immigration authorities at the border port of entry.
• Some travelers are not eligible by law to enter the United States. These include people with certain serious communicable illnesses, criminal records (particularly those involved with drugs), previous deportations from the US, certain visa refusals and other problems with US immigration laws or visas. Such travelers may apply for specially annotated visas; but they may not use the visa waiver program. If they attempt to travel visa-free, they will be refused entry into the United States.
• Visa-free entry does not include those who plan to study, work or remain in the US more than 90 days. Those travelers need visas. If a US Immigration Officer believes that a visa-free traveler is going to study or work or stay longer 90 days, the officer will refuse to admit the traveler. No appeal is possible.

Travelers who use the visa waiver program cannot extend their stay beyond 90 days, and they cannot change their visa status.