American Top 40

American Top 40 (commonly abbreviated to AT40) is an internationally syndicated, independent radio program created by Casey Kasem, Don Bustany, Tom Rounds and Ron Jacobs. Originally a production of Watermark Inc. (later a division of ABC Radio, now Citadel Media), it is now distributed by Premiere Radio Networks in the United States, Canada, Australia, the Philippines, Singapore, China, India, the United Kingdom, Malaysia and several other territories worldwide. It can also be heard on Xbox Live via their iHeartRadio application and on iPhone via the AT40 Mobile App available on iTunes.

Co-creator Casey Kasem hosted the original AT40 from its inauguration on July 4, 1970 until August 6, 1988 and returned to host the revived version from March 28, 1998 to January 3, 2004. (For much of the interim, he hosted the competing Casey's Top 40.) Its other two regular hosts have been Shadoe Stevens (1988–1995) and current host Ryan Seacrest (since 2004) Currently, AT40 with Seacrest airs in two different formats - one for Contemporary Hit Radio and one for Adult Contemporary (formerly Hot Adult Contemporary) stations - with no on-air differentiation other than the charts themselves. Kasem is still heard in two weekly replays of vintage shows from the '70s and '80s, respectively.

As its title implies, AT40 counts down the forty most popular songs in the United States of America, from #40 to #1. The show used Billboard charts in its early years, touting it as "the only source" for the countdown; then, switched to those from Radio and Records upon its late 1990s return. The charts currently are based on data from Mediabase, and published in the Tuesday Edition of USA Today.