An American comic book is a thin periodical, typically 32-pages, containing 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 superheroSuperman. 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, science fiction and romance became popular. The 1950s saw a gradual decline, due to a shift away from print media in the wake of television and the impact of the Comics Code Authority. The late 1950s and the 1960s saw a superhero revival, and superheroes remain the dominant character archetype in the 21st century.
Since the later 20th century, comic books have gained note as collectible items. Comic shops cater to fans, and particularly valuable issues have fetched in excess of a million dollars. Systems of grading comic books have emerged with plastic sleeves ("bags") and cardboard backing ("boards") available to maintain the condition of comic books.
The typical size and page count of comics has varied over the decades, generally trending toward smaller formats and fewer pages. In recent decades, standard comics have been about 6.625 inches (16.83 cm) × 10.25 inches (26.0 cm), and usually 32 pages long.