The First Amendment to the U.S. Constitution is what guarantees the freedom of the press in the United States. Here it is:
"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances."
As you can see, the First Amendment is actually three separate clauses that guarantee not only press freedom but freedom of religion as well as the right to assemble and to "petition the government for a redress of grievances."
But as journalists it's the clause about the press that is most important:
"Congress shall make no law...abridging the freedom of speech, or of the press..."
Press Freedom in Practice
So the Constitution guarantees a free press, which can be extrapolated to include all news media - TV, radio, the web, etc. But what do we mean by a free press? What rights does the First Amendment actually guarantee?
Primarily, press freedom means the news media are not subject to censorship by the government. In other words, the government does not have the right to try to control or block certain things from being published by the press.
Another term often used in this context is prior restraint, which means an attempt by the government to prevent the expression of ideas before they are published. Under the First Amendment, prior restraint is clearly unconstitutional.
