Ornelas v. United States

Law-enforcement departments throughout the United States often make use of a canine (often designated as "K-9") unit. Dogs, with their highly developed sense of smell, can sniff out bombs or drugs that a human inspector might miss, and often a search by a dog with its handler can provide evidence that results in a conviction. Thus drug-detection dogs can often be found at airports, bus terminals, and rail stations--that is, in likely places for the transport of drugs--and they are often used on patrols or sweeps, or for inspections of postal packages.

In United States v. Place (1983), the Supreme Court ruled that a sniff is not a search, and therefore the Fourth Amendment requirements governing the use of drug-sniffing dogs are less stringent than those for human law-enforcement personnel. For instance, if a police officer stops someone for a traffic violation, it is permissible for a canine to sniff the exterior of the individual's vehicle--but only the outside. If a drug-sniffing dog is not present and an officer has reasonable suspicion of drug activity, he or she may detain the vehicle and driver while they wait for the dog to be brought to the scene.