Bubbling Under Hot 100 Singles (also known as Bubbling Under the Hot 100) is a chart published weekly by Billboard magazine in the United States. The chart lists the top songs that have not yet charted on the main Billboard Hot 100. Chart rankings are based on radio airplay, sales, and streams. In its initial years, the chart listed 15 positions, but expanded to as many as 36 during the 1960s, particularly during years when over 700 singles made the Billboard Hot 100 chart. From 1974 to 1985, the chart consisted of 10 positions; since 1992, the Bubbling Under Hot 100 Singles chart has listed 25 positions.
The Bubbling Under Hot 100 Singles chart was first introduced in the June 1, 1959 issue of Billboard, under the name "Bubbling Under the Hot 100". Containing a listing of 15 singles, the chart was described as "the new listing that predicts which new records will become chart climbers." Its first number-one single was "A Prayer and a Juke Box" by Little Anthony and the Imperials (the song would later peak at #81). It would continue to be published in issues of Billboard until August 24, 1985, after which it was discontinued. Prior to its discontinuation, the chart had not been issued in four issues; three from 1974 and one in 1978. However, it returned as a feature in the December 5, 1992, issue of Billboard and continues to the present day. From the end of 1970 to 1985, there was a Bubbling Under the Top LPs album chart paired with the Bubbling Under the Hot 100.
From June 1959 through August 1985, Billboard compiled the chart based on playlists reported by radio station and retail sales outlets surveys. In 1992, Billboard employed updated data capture technology in compiling the chart, using point-of-sale retail information provided by Nielsen SoundScan, input from radio station airplay monitoring provided by Broadcast Data Systems and playlists from small-market systems.
The chart's first issue mentions that a rank position indicates "relative potential to earn an early listing on the Hot 100" and records were ranked starting with number 1. From August 28, 1961, to August 24, 1985, the chart positions were numbered starting with number 101. Songs that have already appeared on the Hot 100 are not included in the Bubbling Under chart as they exit the charts, but may re-enter the Bubbling Under chart at a later date.
Over the years, the chart would undergo several changes and alterations. In the 1960s, the chart included as many as 35 slots; on two rare occasions in 1963 and 1968, the chart contained 36 slots. By the 1980s, the chart contained only 10 slots. On the first issue of its 1992 revival, the chart was renamed to "Bubbling Under Hot 100 Singles" from its previous name, "Bubbling Under the Hot 100". The same issue increased the total number of slots on the chart to 25 and the chart numbering began with the number 1.
Several reference books on the history of the Billboard "Bubbling Under" charts have been published by chart statistician Joel Whitburn's company Record Research. The latest book to be published by the company was 2005's Bubbling Under the Billboard Hot 100: 1959-2004 (ISBN 978-0-89820-162-8). Whitburn's book Top Pop Singles, 12th Edition (ISBN 978-0-89820-180-2), covers all Billboard Hot 100 and Bubbling Under Hot 100 Singles chart entries from 1955 to 2008.