You need a visa in most cases if you are entering the USA. It does not matter when the visa expires, as long as it is good on entry.

There are a few exceptions. You do not need a visa if:

• You are from a visa waiver country and plan to come to the US as a tourist or business visitor for no more than 90 days. Note that effective October 1, 2003, you must also have a machine-readable passport (MRP). If you do not have a machine-readable passport, you must apply for a visa even if you are from a visa waiver country. Department of State announced that eventually, they will require passports with biometric identifiers.
• You are a Canadian citizen without a criminal record, and plan to enter the USA as a non-immigrant. There are a few non-immigrant categories where even Canadian citizens need a visa, though (K, the fianc頶isa, is one of them).
• You leave the USA for a short trip (30 day maximum) to Canada or Mexico. If you are an F-1 student, you can also use the same procedure to travel to Caribbean islands. This is known as Automatic Revalidation. There are exceptions to this rule:
  • Your I-94 needs to be still valid when you return.
  • If your status is K-1 or K-2, this is not available.
  • You must have a visa in your passport. The visa can be expired or of a different category, but must be the one you used when entering the USA. Of course, visa waiver tourists are exempt from this requirement.
  • You cannot be a citizen or national of one of the so-called terror-sponsoring nations (Iran, Iraq, Syria, Lybia, Sudan, North Korea and Cuba).
  • You must not have left the country you travelled to (that is, you cannot go to Guatemala from Mexico, or go from Canada to your home country).
  • You must not apply for a US visa in Canada or Mexico. If you do, Automatic Revalidation is no longer available, and you need to have the actual correct type of visa.