The Top Forty or Top 40 is a music industry shorthand for the currently most-popular songs in a particular genre. When used without qualification, it typically refers to the best-selling or most frequently broadcast pop music songs.

The Top 40, whether surveyed by a radio station or a publication, was a list of songs that shared only the common characteristic of being newly released. Its popularity coincided with the rapid changes in recording technology in the fifties and sixties. In 1954, the recording industry agreed upon a standard recording format for higher fidelity music, so any new record player could play any new record. Also that year, new single records were released on 45 rpm records and the Top 40 thereafter became a survey of the popularity of these records. Tape recording had become perfected, allowing artists more freedom as they composed songs, especially novelty songs .

In the heyday of Top 40, between two and seven songs would enter and leave the survey. Chart runs could range from a week or two, to several months. Generally, only the biggest hits of a given year would remain on the charts for fifteen weeks or more. Fans would associate seasons and semesters with the songs they remembered from the juke boxes or the radio.

Jingles, contests, listener dedications, news updates, traffic reports, and other features were designed to make Top 40 radio particularly attractive to listeners. By early 1964, the era of the British Invasion, Top 40 radio had become the dominant radio format for North American listeners and quickly swept much of the Western world, being brought into the United Kingdom by offshore stations such as Wonderful Radio London, and later adopted by BBC Radio 1. Some stations tried extremely "tight" radio playlists, going with the Top 30 or even the Top 20 songs, but most industry experts felt that listener fatigue would set in more quickly with smaller lists. Top 40 quickly became the dominant radio format of the 1960s, 1970s, and 1980s, although as music formats began to fracture, most stations began to specialize in certain restricted kinds of popular music, usually playing specific types of rock such as mainstream, the so-called "soft rock", or other music charted by radio industry trade publications.

Other lists of hit songs may include a different number of entries, such as a "Top 50" or "Top 100".

By the late eighties and early nineties, the 45 rpm record would become extinct and other means would be used to evaluate the popularity of new songs. As a result, chart runs for songs tend to be longer than they were in the sixties and seventies.

The current top songs are tracked by a variety of trade publications, such as:

• Argentina Top 40, the ranking of the top 40 songs charting in the official Argentine chart.
• Top 40 Mainstream, a chart of current popular songs in the United States, compiled by Billboard Magazine and its various predecessors
• Radio & Records magazine, recently purchased by Billboard Publications
• Top Latino, the Latin American Music Chart.
• UK Top Forty, a British version of the same concept.
• Dutch Top 40, the Dutch music chart.
• Ö3 Austria Top 40, the official Austrian music chart.
• Los 40 Principales, in Spain.
• The Net 40, a worldwide user generated Top 40 show.

Radio programs that highlight currently popular songs also refer to the "Top 40":

• Top 40 (radio format), a radio format based on playing the top 40 popular songs of the week (may or may not be based on Billboard's chart), also known as "contemporary hit radio" or "CHR"
• American Top 40, a weekly radio syndicated countdown radio show originally created and hosted by Casey Kasem from 1970 to 1988 and again from 1998 to 2004, and hosted by Ryan Seacrest, as of 2004.
• EHR Top 40, European Hit Radio Top 40 aired in Latvia and Lithuania.

Further reading

• Durkee, Rob. "American Top 40: The Countdown of the Century." Schriner Books, New York City, 1999.
• Battistini, Pete, "American Top 40 with Casey Kasem The 1970s." Authorhouse.com, January 31, 2005. ISBN 1-4184-1070-5.
• Douglas, Susan, "Listening In: Radio and the American Imagination," New York: Times Books, 1999.
• Fong-Torres, Ben, "The Hits Just Keep On Coming: The History of Top 40 Radio", San Francisco: Backbeat Books, 1998.
• MacFarland, David, "The Development of the Top 40 Radio Format", New York: Arno Press, 1979.
• Fisher, Mark, "Something in the Air: Radio, Rock, and the Revolution That Shaped a Generation", New York: Random House, 2007.
• Goulart, Elwood F. 'Woody', "The Mystique and Mass Persuasion: Bill Drake & Gene Chenault’s Rock and Roll Radio Programming[1]", 2006.