The recurring slogan "Who Watches the Watchmen?" is most often seen as graffiti throughout the story. It is never seen complete; it is always obscured by a foreground object or person, or unfinished by the graffiti artist, however it is always discernible. The slogan is a reaction by the public to a lack of law enforcement protection during a national police labor strike prior to the passage of the Keene Act. Police protection of society has vanished and the costumed crimefighters of the story engage in crowd control. Riots have broken out in New York City and Washington, D.C., and many ordinary citizens have taken to the streets to protest the intervention of costumed heroes in law enforcement.
After the last chapter of the graphic novel is finished, the slogan appears in the original Latin (found in the work of the Roman poet Juvenal from his Satires (Satire VI, lines 347–348): "Quis custodiet ipsos custodes."
In the final chapter of Watchmen, following the reconstruction efforts after the alien monster's appearance, a similar phrase can be seen graffiti'd on a wall that Seymour walks past. Again, the tag is partly concealed, but enough is visible to make it clear what it says: "Watch the Skies".