Since 1956, the term "Top 40" has referred to both a ranking of the best selling music singles and to a radio (see entry under 1920s—TV and Radio in volume 2) format that features those songs. For a song to be included in the list, it must be among the top 40 records in terms of record sales and radio station airplay nationwide. As they grow or fall in popularity, records can move up or down the list. The length of time a record stays on the Top 40 list indicates a hit record. For better or worse, Top 40 has proved to be an influential format in the world of radio.

Although some stations had featured lists of songs as far back as the 1930s, the Top 40 as a standard format began in the Midwest in the early 1950s. In the early 1950s, before television (see entry under 1940s—TV and Radio...