The first person you meet on arrival in the United States -- whether you come by air, land, or sea -- will be an officer of Customs and Border Protection, or CBP. The officer will inspect your passport and documents, looking for verification that you've been given permission to enter the U.S., as well as any information that might prevent you from doing so. Have all your visa paperwork ready. Even if your visa and intentions are valid, if the officer finds a problem or believes you're lying, you can be refused entry.
For the protection of the United States, people with histories of criminal or terrorist activities, drug abuse, infectious medical problems, or certain other characteristics will never be allowed a visa or green card. These characteristics are known as the grounds of inadmissibility. Even if you fall into one of the grounds of inadmissibility, there are a few ways you may still qualify for a green card or U.S. entry.