Effective January 1, 2003 - Entry Requirements Change - Passports are now mandatory.  

U.S. Passports must be valid at least 6 months or longer beyond the dates of your trip.   
Identification / Entry Requirements:

   For US citizens, a valid passport. Upon entry, each tourist is issued a 90-day tourist visa. If you plan to stay beyond the 90 days, make sure you leave the country for 72 hours and then re-enter for a new 90 day visa or receive special permission / work visa before you arrive. It's also a good idea to have several copies of the picture page of your passport with you when you travel.
What immunizations are required before I travel to Costa Rica?

     No immunizations are required or recommended, but consult your general physician before you travel. As with any travel, check to make sure your inoculations are up-to-date. When was your last tetnas shot?