If you have been entering the U.S. as a Visa Waiver traveler (WT/WB)on a non-machine readable passport, the rules are changing. As of October 1 you will need to either get a machine-readable passport (MRP) or get a US non-immigrant visa (B1/B2).
How do I know if I was on a visa waiver before?
Do you have a colorful sticker from the US embassy in your passport? Common visas include F1 for students and H1B for temporary workers. If you used a two-sided green form you entered VWP, if you used a one-sided white one, you did not.
What are the Visa Waiver countries?
Countries participating in the Visa Waiver Program (VWP) are: Andorra, Australia, Austria, Belgium, Brunei, Denmark, Finland, France, Germany, Iceland, Ireland, Italy, Japan, Liechtenstein, Luxembourg, Monaco, Netherlands, New Zealand, Norway, Portugal, San Marino, Singapore, Slovenia, Spain, Sweden, Switzerland, and the United Kingdom.
How do I know if I have a MRP?
Look at the bottom of the biographical page of your passport. Under the photo there may be two or three lines of ASCII text (you can probably identify your name) and a bunch of carrots (e.g. >>>>>JOE>PUBLIC>>>>). If you see this, you have an MRP. If you don’t see that, you probably need to seek out a visa or a new passport.
[UPDATE Sep 22 2003: Rumors coming down from On High are saying that this requirement has been suspended until October 2004. So far we haven’t had any confirmation of this.]