Every day, CBP welcomes more than 1.1 million international travelers into United States at its land, sea, and air ports!
When travellers come through US "Customs," they must show their passport to a Customs officer. Here is a photo gallery of Customs Officers working at an airport. Listen to a conversation between a Customs Officer and a woman re-entering the United States.
CBP has information sheets about what a traveller should "Know Before You Go" outside of the United States.