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. According to legend, the Top 40 radio format was created by Todd Storz in Omaha in 1955, after observing customers at a bar playing the same handful of songs on a jukebox over and over again. Storz realized that treating the radio station's playlist as a jukebox would ensure that listeners would always hear popular songs, regardless of what time of the day they tuned in, if the station made an effort to play a smaller group of songs. The music ranged anywhere from country & western to pop music, rock & roll, even instrumentals and novelty songs.

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".

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

• 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
• 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.

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.