Passports
Note: Under the Western Hemisphere Travel Initiative (introduced 23 January 2007), US citizens returning directly from a US territory do not need to present a passport to re-enter the USA. For further details about the Western Hemisphere Travel Initiative, visit the website of the US Department of State: http://travel.state.gov/travel/cbpmc/cbpmc_2223.html.
Passport Note
The passport and visa requirements for entering Puerto Rico are the same as for entering the USA. See Passport/Visa in the USA section.