U.S. and Canadian citizens are required to present a passport, or a birth certificate with a photo ID (a drivers license, for example) showing the same name as on your birth certificate, in order to enter St. Kitts and Nevis.  Where applicable, documentation of a legal name change (for example, a marriage certificate) must also be presented to show why the name on your photo ID does not match your birth certificate.   All other visitors must present a passport. 

Visitors may stay for a maximum of six months, but extensions may be requested at the immigration office in Charlestown, Nevis.  Visitors must enter with a return ticket.  There is a departure tax when leaving the island of $20.50 USD or $54 EC, which includes an environmental levy and airport security tax. 

For U.S. Citizens, it's important to note that in order to re-enter the United States you will be required to show a valid current passport as of January 1, 2007.  This applies to all re-entries made by air (landing at any U.S. Airport from any non-U.S. origination).  Land border crossings and Sea re-entries (disembarking from a cruise ship) will require a valid current passport as of Janaury 1, 2008, unless Congress extends the deadline for land/sea border crossings.   So you could land in Canada or Mexico and then drive across the border, but you may as well just get a passport.  In the long run, it's cheaper.

Additional Resources:

• US Department Of State (information on US Passports & travel requirements)
• For assistance in getting Passports and Visas Click Here