What has to happen for a book to be banned?

Books should not be banned from anything. Read Fahrenheit 451 if you still disagree well then just don't read it or let your kids read it, simple, I believe.

In America, it's all due to the errors of political correctness. One person can claim to be offended by a book without any reasonable explanation, and an entire school board will immediately ban it for fear of being labeled a racist or bigot. (For example: Huckleberry Finn)

The American Library Association has a website that lists books that have been banned somewhere in the United States. As the previous contributors noted, it's almost always school boards that do so because they are over-reacting to a complaint from someone who has probably not even read the book in the first place. The fact that To Kill A Mockingbird and Huckleberry Finnare always on the ALA list, year after year, condemned as racist books in spite of the fact that they are absolutely anti-racist in every possible respect, is a sad commentary on American illiteracy and many school boards' lack of integrity.