Writing for the Court, Justice John Paul Stevens first addressed the narrow question facing the court, noting that the Court “proceed[ed] on the assumption that the officer conducting the dog sniff had no information about [Caballes] except that he had been stopped for speeding.” He added that the initial seizure of Caballes for speeding was lawful and that the only question facing the court was whether the subsequent use of the drug-sniffing dog was permissible.

Stevens began his analysis by establishing that “a seizure that is lawful at its inception can violate the Fourth Amendment if its manner of execution unreasonably infringes interests protected by the Constitution.” Regarding traffic stops, Stevens noted that “[a] seizure that is justified solely by the interest in issuing a warning ticket to the driver can become unlawful if it is prolonged beyond the time reasonably required to complete that mission.” He further elaborated that “conducting a dog sniff would not change the character of a traffic stop that is lawful at its inception and otherwise executed in a reasonable manner, unless the dog sniff itself infringed respondent’s constitutionally protected interest in privacy.” Stevens emphasized that there is no “legitimate” interest in possessing contraband, so “governmental conduct that only reveals the possession of contraband ‘compromises no legitimate privacy interest.’” He explained that “[t]his is because the expectation ‘that certain facts will not come to the attention of the authorities’ is not the same as an interest in ‘privacy that society is prepared to consider reasonable.’” Therefore, Stevens found that “the use of a well-trained narcotics-detection dog—one that ‘does not expose noncontraband items that otherwise would remain hidden from public view,’… during a lawful traffic stop generally does not implicate legitimate privacy interests.”

Regarding Caballes’ case, Stevens highlighted that the dog sniff was conducted on the exterior of his vehicle while he was lawfully seized for a speeding, meaning that “[a]ny intrusion on respondent’s privacy expectations does not rise to the level of a constitutionally cognizable infringement.” Stevens emphasized the distinction between legitimate interests protected by the Fourth Amendment and those that are not, writing “[t]he legitimate expectation that information about perfectly lawful activity will remain private is categorically distinguishable from respondent’s hopes or expectations concerning the nondetection of contraband in the trunk of his car.” Ultimately, Stevens concluded that “[a] dog sniff conducted during a concededly lawful traffic stop that reveals no information other than the location of a substance that no individual has any right to possess does not violate the Fourth Amendment.”