Adult contemporary music (AC) is a broad style of popular music that ranges from lush 1960s vocal music to predominantly ballad-heavy music with varying degrees of rock influence. AC radio plays mainstream music excluding hip hop, hard rock, youth orientated R&B, some teen pop music (artists such as Taylor Swift, Hilary Duff, and Miley Cyrus may be included) and rhythmic dance tracks (though since the 2000s, some of these have begun to be included), which is intended for a more adult audience. Radio stations playing this format will often target 18-54 year-olds, the group most valued by advertisers. Over the years AC has spawned several sub-genres: "hot AC", "soft AC" (also known as "lite AC"), "modern AC", "urban AC", "rhythmic AC", "smooth AC" (i.e., smooth jazz), and "Christian AC" (i.e., a softer type of Contemporary Christian music). Some radio stations play only hot AC; some play only soft AC; and some play both. Thus it is not usually considered a specific genre of music, since it is merely an assembly of selected tracks of musicians of many different genres.

Early radio stations played top-40 hits, theoretically regardless of genre although most were in the same genre until the mid-1970s when different forms of popular music started to target different demographic groups, such as disco vs. hard rock. This evolved into specialized radio stations that played specific genres of music, and generally following the evolution of artists in those genres. One big impetus for the development of the AC radio format was the fact that when rock and roll music first became popular in the mid-1950s, many more conservative radio stations wanted to continue to play current hit songs while shying away from rock. These stations also frequently included older, pre-rock-era adult standards and big band titles to further appeal to adult listeners who had grown up with those songs. In those days, middle of the road or "MOR" was the formatic term used to describe such stations, which included powerhouse broadcasters like WJR in Detroit, WGN and WBBM in Chicago, KGO in San Francisco, WNEW-AM in New York, and KMOX in St. Louis. Billboard magazine first published an adult-contemporary music chart in 1961, although it was not until 1979 that the chart took on the name "Adult Contemporary".

While most popular MOR stations were, like Top 40 stations of the day, on the AM dial, another big impetus for the evolution of the AC radio format was the popularity of easy listening or "beautiful music" stations, stations with music specifically designed to be purely ambient, listened to while at work or otherwise in the background. These stations were largely found on the FM dial alongside classical music stations because the music they played sounded better on FM. Whereas most easy listening music was instrumental, created by relatively unknown artists (except for occasional MOR vocal hits), and rarely purchased, AC was an attempt to create a similar "lite" format by choosing certain tracks (both hit singles and album cuts) of popular artists. The growth of AC was also a natural result of the generation that first listened to the more "specialized" music of the mid-late 70s growing older and not being interested in the heavy metal, rap and hip-hop music that a new generation helped to dominate the top-40 charts (this effect has also altered the oldies format; as there are now two kinds of oldies stations, those who will not play songs from after the early 1970s vs. those who will play songs up to the early 1980s while still having occasional pre-1964 songs in rotation). Fans of harder rock music often derogatorily referred to AC stations in the early days of the format as "chicken rock".