Freedom of speech is the liberty to speak and write without fear of government restraint. It is closely linked to freedom of the press. In the United States both freedoms—commonly called freedom of expression—are protected by the 1st Amendment to the Constitution. The amendment provides that "Congress shall make no law abridging the freedom of speech, or of the press."

Most other Western countries guarantee freedom of speech, either in their constitutions or by legislative enactment. All countries, however, limit manifestations of free speech that are regarded as threatening the civil order or as obscene or slanderous. The extent to which speech is regarded as threatening or slanderous and the way in which limits are imposed are critical factors in determining the degree of free speech in a society.

The quest for free speech has a long, turbulent history. It has been one fundamental aspect of the individual's developing relationship both to the state and to society. Until the 17th century various forms of censorship of free speech were common; they were contested principally within the framework of larger issues of political and religious conflict. In England in the 17th century, however, freedom of speech began to assume its own importance. John Milton wrote in his Areopagitica (1644): "Give me the liberty to know, to utter, and to argue freely according to conscience, above all liberties." Philosophers such as John Locke, Voltaire, and, later, John Stuart Mill took up the cry. The right to freedom of speech was recognized in the English Bill of Rights (1689), the French Declaration of the Rights of Man (1789), and the U.S. Bill of Rights (1791). It was to become an integral part of constitutional law even in countries that do not in reality permit free speech. And it gained international recognition with the United Nations' proclamation of the Universal Declaration of Human Rights (1948).

In the United States freedom of speech and the constitutional limits to it have been defined, in practice, by rulings of the Supreme Court. Originally the free-speech guarantee of the 1st Amendment applied only to acts of Congress. In the 20th century, however, the Supreme Court began to interpret the due process clause of the 14th Amendment to mean that the states are also bound by the provisions of the 1st Amendment. This was one of the major new doctrines of Gitlow v. New York (1925).

Restrictions on freedom of speech have occurred most often in time of war or national emergency. The Alien and Sedition Acts of 1798 were the first incursions by Congress on this freedom. These laws were never tested in the courts and were allowed to expire after several years.

The first clear-cut test came over the Espionage Act (1917) passed by Congress during World War I; this act made it illegal to interfere with the recruitment or drafting of soldiers or to do anything adversely affecting military morale. In Schenck v. United States (1919) the Court upheld the conviction of a socialist indicted under the act on the ground that freedom of speech is not absolute. Justice Oliver Wendell Holmes, Jr., delivered the Court's unanimous opinion; he argued: "The most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre and causing a panic. . . . The question in every case is whether the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent."

The "clear and present danger" doctrine became one of the tests the Court applied to subsequent cases involving freedom of speech. Another test, which placed more restrictions on individual expression, was whether an expression had a tendency to lead to results that were bad for the public. In Gitlow v. New York the Court held: "a State in the exercise of its police power may punish those who abuse this freedom by utterances inimical to the public welfare, tending to corrupt public morals, incite to crime, or disturb the public peace. . . ." Gitlow had been indicted under a New York State law prohibiting the advocacy of the overthrow of the government by force or violence.    

In 1940, Congress enacted the Smith Act. It declared unlawful the advocacy of overthrowing the government by force or violence. Eleven leaders of the Communist party were convicted under the Smith Act and appealed on the ground that it was unconstitutional. The Court upheld the constitutionality of the act in Dennis v. United States (1951). The ground was not, however, "clear and present danger." Instead, the majority adopted a standard put forward by Justice Learned Hand; "in each case," he said, courts "must ask whether the gravity of the ‘evil,’ discounted by its improbability, justifies such invasion of free speech as is necessary to avoid the danger." This standard has been called the "clear and probable danger" test. The approach, modified by other cases, has been termed the "balancing" test. When applying the balancing approach, the Supreme Court strives to strike a balance between the value of liberty of expression and the demands of order in a free society. Many critics of the balancing approach have contended that a balance is rarely struck; in most cases in which it is involved, they say, society prevails over the individual. The USA Patriot Act, passed in the wake of the September 11, 2001, terrorist attacks, provoked a renewal of this argument. Some of its provisions, including authorization of the Federal Bureau of Investigation to seek the library records of individuals, were seen by many as a clear violation of free-speech rights.

The preferred-freedoms approach has been important in constitutional law since World War II. The position was originally set forth by Justice Harlan F. Stone. This approach stresses that the civil liberties have a preferred position among other constitutional values since they are requisite to a democracy. Under this concept the burden lies largely with the government to prove that when a particular freedom is exercised, a clear and present danger exists.

Some justices have tended to see freedom of speech as nearly an absolute right; notable among them are Hugo Black and William Douglas. The difficulty of the absolute approach to free-speech issues was shown (1977–78) when a group of American Nazis sought to hold a rally in Skokie, Ill. They were denied a permit by the municipality on the ground that a Nazi rally would incite hostility in the largely Jewish population, which included many survivors of Nazi concentration camps. Lawyers for the American Civil Liberties Union (ACLU) represented the Nazis; they argued that the Skokie laws limiting public demonstrations were unconstitutional. A U.S. court of appeals agreed with the ACLU. Many Americans, however, were outraged at the defense of those they considered the enemies of free speech.

The judicial interpretation of the right of free speech has yet to produce a clear definition of what is permissible. Insofar as seditious speech is concerned, the courts have held language permissible so long as it does not tend to incite the violent overthrow of the government. In other free-speech areas, the courts have also had to consider the various interests of society and the requirements of the Constitution; such areas include obscenity and pornography, "fighting words," picketing or demonstrating, symbolic speech, and loyalty oaths. Flag-burning, which is a form of symbolic speech, has proved a particularly contentious issue. The Supreme Court has overturned state and federal laws prohibiting desecration of the flag, arguing that such action is protected under the 1st Amendment.

In several treaties and conventions, European, Latin American, and international organizations have pledged respect for fundamental freedoms and human rights. The practice of free speech, however, is consistent only in the democracies of Western Europe, English-speaking countries, and Japan. Authoritarian, theocratic, totalitarian, and dictatorial governments such as those in Communist countries, most of the Arab world, much of Africa and Asia, and, until recently, most of Latin America and Eastern Europe commonly suppress freedom of speech. Many developing countries cite the task of economic progress as reason for denying this liberty.

The 1980s and early 1990s were marked by a strong trend toward greater free speech in the world. Mikhail Gorbachev's introduction of glasnost ("openness") in the USSR set in motion the changes that brought the collapse of the Soviet system in 1991. Most of Latin America's military regimes and the Communist governments of Eastern Europe have been replaced by fledgling democracies. This trend, however, has frequently resulted in political instability and renewed repression. China's prodemocracy movement, for example, was militarily crushed in 1989, marking the return of full dictatorial rule. (See Tiananmen Square Massacre.) Almost two decades later, China's human rights policies, including its suppression of free speech, were again under an international spotlight when the 2008 Olympic Games were held in Beijing.

The growth of the Internet at the turn of the 21st century created a remarkable forum for participatory mass speech. It has given individuals the ability to exchange ideas and express themselves with a freedom never before enjoyed. In Reno v. American Civil Liberties Union (1997), the U.S. Supreme Court gave its first ruling on efforts to regulate that freedom. It struck down the 1996 Communications Decency Act, a federal law that outlawed "indecent" communications online. The Court declared that the Internet deserved at least as much 1st Amendment protection as that afforded to books, newspapers, and magazines. Nevertheless, governments and other powerful institutions have sought ways to censor information on the Internet. Some governments use filtering software to block access to certain Web sites; the Chinese government has been very effective at this. In 2007, during an upsurge of unrest in Myanmar, the government of that country cut off all Internet access.