Dexter structures his killing around "The Code of Harry", a body of ethics and procedures devised by his adoptive father Harry (who was a Miami cop) to make sure Dexter never gets caught and ensure that Dexter kills only other killers. Harry also trained Dexter in how to interact convincingly with other people despite being a psychopath, since the murder of his biological mother, Laura Moser, did in fact turn Dexter into a serial killer. As an adult, Dexter has largely escaped suspicion (with some exceptions) by being genial and generous and maintaining generally superficial relationships. However, his attachment to his sister Deb, his significant other Rita, his stepchildren and (later) his biological son have all complicated his double life and made him question his need to kill.

Dexter has followed The Code religiously to satisfy his Dark Passenger (what he calls his urge to kill). Like many serial killers, he keeps trophies; before dispatching a wrongdoer, Dexter makes a small incision on their right cheek with a scalpel and collects a blood sample which he preserves on a blood site.

Dexter's victims must be killers themselves who have murdered the undeserving and will likely do so again. Dexter must also have proof that the criminals are guilty. Most importantly, Dexter must never get caught. Flashbacks throughout the series show Harry, who died several years before, instructing Dexter in how to appear normal and cover his tracks.