As written in Wikipedia: American Top 40 is an internationally-syndicated, independent radio program created by Casey Kasem, Don Bustany, Tom Rounds and Ron Jacobs.
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. Its other two regular hosts have been Shadoe Stevens (1988–1995) and current host Ryan Seacrest (since 2004). As of March 2009, AT40 with Seacrest airs in two different formats – one for Contemporary Hit Radio and one for 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, 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 Sunday Edition of the USA Today.
Currently runs Saturday mornings 6am-10am and Sunday afternoons 12noon – 4pm.