Die Drei ??? The Three Investigators (original title: The Three Investigators; alternative form: The Three Detectives) is a series of young adult books. It originates from the United States and has been translated into many languages. In Germany, the book series began in 1968 with translations of the English-language originals and has been continued since 1993, after the series was discontinued in the United States, with stories by German-speaking authors. Since 1968, over 16.5 million books in the series have been sold across Germany. Since 1979, it has also been successfully produced as a radio play series, which has sold more than 45 million audio recordings in Germany, as well as around two million games, detective game boxes and fan articles.