In Japan, the history of radio drama is started with the first radio broadcast in 1925. Some consider the first radio drama in Japan to be "Kirihitoha (桐一葉, The Falling Paulownia Leaf)" which was a radio broadcast of a stage play. Others consider the Japanese translation of Richard Hughes's "Danger" or Tankō no Naka (炭坑の中, Down the coal) to be the first true radio drama to be broadcast in Japan. The Japanese public broadcaster, NHK, also had a special radio drama theatrical company that is the origin of the seiyū phenomenon in Japan that continues to this day.
In the 1950s, authors like Shinichiro Nakamura (中村真一郎), Kiyoteru Hanada (花田清輝), and others who belonged to the "junbun gaku (純文学, "pure literature movement") penned many experimental radio dramas. These radio dramas caught the attention of various Eastern European countries, and as a result, these works were translated and rebroadcast. As with most countries, radio drama broadcasts have become less common after the advent of television.
In Japan today, it is common for popular television dramas, "Light novels", manga series, anime series or video games to have main plot lines, plot continuations, sequels or small side stories released in the form of audio dramas.
These audio dramas are alternatively called drama CDs (ドラマCD), radio dramas (ラジオドラマ), or sound dramas (サウンドドラマ).
Today they are released on Compact Disc, however in the past they were released on vinyl and audio cassette.) Before the advent of videocassette recorders, drama recordings were the only way to revisit an animated television series. Recordings often featured recapitulations of plotlines along with theme songs from anime series. This is still employed by current audio dramas, for example the first Sailor Moon audio drama CD has the characters getting into a shiritori battle with Zoisite featuring the names of minor characters and place settings. 
Audio drama plotlines may also be re-used in other media. An example of this are audio dramas like Benitokage from Sakura Taisen which was later produced as a stage show; then used as a basis for an episode in direct-to-video anime release. Another example would be how the audio drama Getter Robo : The Moon Wars was used as the starting point for the OVA series Change!! Getter Robo: The Last Day of the World
Most modern audio dramas consist of either side stories or parody stories, though an audio drama may be both. Side stories are usually extensions of main plotlines such as plotlines that were featured in manga that have not appearead in an anime. Parody stories feature characters getting in to humorous predicaments or scenes that may be too risqué for television. For example, one Sailor Moon audio drama featured a scene where Haruka Tenoh filled in at a gay bar.
Recent trends in merchandising anime shows have had audio dramas come out as pretexts for the development of anime series and can substantially precede the appearance of an anime version. Sometimes they are released before an animated version of an anime series in order to introduce fans to the characters and Seiyū. (An example of this was the manga series Angel Sanctuary which had a drama CD come out well before its direct-to-video anime release.)