Puerto Rico is a U.S. territory, so travel from the mainland United States is domestic. No passport or customs are required for U.S. citizens flying from the mainland — your regular ID is fine.
By Air
Luis Muñoz Marín International Airport (SJU) in San Juan is the island's main gateway and one of the busiest airports in the Caribbean. Nonstop flights operate from Atlanta, Boston, Chicago, Dallas, Houston, Los Angeles, Miami, New York, Orlando, and many other U.S. cities. Major carriers include American, Delta, JetBlue, Southwest, Spirit, United, and Frontier. Flight times run roughly 3h45m from New York, 2h30m from Miami, and 4h30m from Chicago.
Rafael Hernández Airport (BQN) near Aguadilla on the west coast receives flights from Orlando and several northeastern U.S. cities, useful if you're heading to Rincón or the west coast and want to skip San Juan entirely. Mercedita Airport (PSE) in Ponce has limited service, mostly from the U.S. East Coast.
International travelers arriving from outside the United States go through U.S. customs and immigration in Puerto Rico.
By Sea
Several major cruise lines call at San Juan, which is the Caribbean's busiest cruise port. San Juan also serves as a homeport, so some cruise itineraries begin and end here.