Obscenity, as defined by the U.S. Supreme Court, conduct or material appealing solely to lewd tastes and “lacking serious literary, artistic, political, or scientific value.” Obscenity includes pornography (indecent writing or pictures), indecent exhibitions, and indecent language in a public place. The federal government prohibits the mailing, interstate sale and distribution, and broadcast of obscene material. States also have laws against obscenity.

In the case of Roth v. the United States (1957), the Supreme Court ruled that obscenity is not protected by the free speech guarantee of the First Amendment. Application of principles put forth in this case came to be known as the “Roth test.” National standards of decency were now applied to questionable material. Accordingly, material “utterly without redeeming social value” was regarded as obscene. The Roth test was revised in Miller v. California (1974), in which the court ruled that obscenity is defined by community standards of decency and “serious” social value.

Obscenity laws at one time or another have prohibited the sale of some distinguished literary works, notably James Joyce's novel Ulysses (1922). Other means of expression, such as songs and works of art, have also been suppressed because they were considered obscene. In 1990, for example, a rap music group known as 2 Live Crew was arrested and charged with performing indecent acts during a concert, and an exhibition of Robert Mapplethorpe's photography was shut down by city officials in Cincinnati on the grounds that it offended community standards of decency.