543 U.S. 405 (2005)
Illinois v. Caballes is a foundational Fourth Amendment case defining the constitutional limits on the use of drug-detection dogs during routine traffic stops.
Does the Fourth Amendment require reasonable suspicion to justify using a drug-detection dog to sniff a vehicle during a lawful traffic stop when the sniff does not prolong the stop?
A canine sniff conducted during a lawful traffic stop that does not prolong the stop is not a Fourth Amendment "search" and therefore does not require reasonable suspicion. Individuals have no legitimate privacy interest in possessing contraband, and a dog sniff that reveals only the presence of contraband does not implicate constitutionally protected privacy interests.
No. The use of a drug-detection dog to sniff the exterior of a vehicle during a lawful traffic stop, so long as it does not extend the duration of the stop, does not violate the Fourth Amendment and does not require reasonable suspicion.
Caballes cements the principle that a dog sniff of a vehicle's exterior during a lawful, non-extended traffic stop is outside the Fourth Amendment's definition of a "search." It is essential reading together with Whren v. United States (validity of pretextual stops), Indianapolis v. Edmond (limits on general crime-control checkpoints), Kyllo v. United States (technology that reveals lawful intimate details is a search), Florida v. Jardines (canine sniff at a home is a search), Florida v. Harris (dog reliability proof), and Rodriguez v. United States (any prolongation of a stop for a dog sniff requires reasonable suspicion). For practice, Caballes directs officers and courts to ask two core questions: Was the stop lawful? And did the canine sniff add time to the stop? If the answer to the first is yes and the second is no, Caballes authorizes the sniff without reasonable suspicion.