An American comic book is a thin periodical containing primarily comics content. While the form originated in 1933, American comic books first gained popularity after the 1938 publication of Action Comics, which included the debut of the superhero Superman. This was followed by a superhero boom that lasted until the end of World War II. After the war, while superheroes were marginalized, the comic book industry rapidly expanded, and genres such as horror, crime, and romance became popular.