The sole justification for a Terry frisk is the protection of the police officer or others nearby. The arresting officer must be able "to point to particular facts from which he reasonably inferred that the individual was armed and dangerous." Good faith on the part of the officer, in itself, is not enough to justify a Terry frisk.

Actual observation of illegal activity among a defendant's companions, combined with the defendant's suspicious behavior and the officer's knowledge that they are in a high crime area known for drug activity can provide reasonable suspicion to justify an investigatory stop. However, the absence of any specific, articulable facts establishing that a defendant was armed and dangerous renders a subsequent frisk unlawful.

A seizure occurs when a reasonable person would believe that he or she is not free to leave. Police officers act legally in asking for and examining an airline ticket and driver's license of an airline passenger whose characteristics fit a "drug courier profile." But – in absence of probable cause – the officers act illegally when they identify themselves as narcotics agents, inform the passenger that they are suspected of transporting narcotics, and ask the passenger to accompany them to police room, while retaining the passenger's ticket and driver's license and without indicating in any way that the passenger is free to depart.

A person who has been expressly informed by a police officer that he or she is being "stopped" as part of an investigation into drug activity and who has his or her background checked for prior criminal activity by that officer via police radio would not reasonably feel free to terminate the encounter with the officer and walk away. Thus, such a person is the subject of an investigative detention by the police officer.

Once a traffic stop has ended, an officer's has no authority to order either driver or occupant from the car. If the officer requests that the occupants exit the vehicle that show of authority may constitute an investigatory detention that must be supported by a renewed showing of reasonable suspicion. Moreover, the observation during a routine traffic stop of head and shoulder movements of the rear seat passenger in a motor vehicle, coupled with the officer's conclusion that the passenger appears "very, very nervous," does not provide sufficient basis for the detention and search of the passenger.

An individual's act of walking away from police officers in a "high crime area" is manifestly insufficient to justify an investigative detention of that individual.

Where the source of the information given to police officers is unknown, independent corroboration of the essential allegation can give rise to reasonable suspicion justifying a stop and frisk. Information provided by a radio call may not include a range of details or indicate future behavior, but officers may confirm the location and description of a suspect, and then observe suspicious and furtive behavior that, in their experience, is consistent with the dealing of narcotics. Such suspicious conduct in an area associated with criminal activity provides independent corroboration of the essential allegation and, thus, suggests that criminality may have been afoot.

A tip from an anonymous pedestrian that there is "a tall man in the park brandishing a weapon" does not provide a police officer with reasonable suspicion of criminal activity. When the identity and veracity of an informant are unknown, there is no objective basis for an officer to conclude that the information provided by the informant is accurate or reliable. Thus, the officer needs "something more" than the tip itself to effectuate a Terry stop of an individual who might possibly be the subject of the tip. Specifically, independent corroboration of an individual's involvement in criminal activity is necessary for reasonable suspicion.

Before the police may undertake a stop and frisk on the basis of an anonymous telephone tip of a man with a gun, the police must establish that they have a reasonable suspicion that the individual is involved in or about to commit a crime. If the tip contains sufficient information, the police can merely corroborate sufficient details of the tip. Otherwise, the police must investigate further by means not constituting a search and seizure. If, as a result, they acquire sufficient information to give rise to a reasonable suspicion that the individual is armed and dangerous, they may then initiate a Terry stop.

Officers have reasonable suspicion to justify the pursuit of a fleeing suspect when they have made past drug arrests in the same area, they observe an attempted hand-off of an unidentified object by the suspect, the suspect appears nervous when officers make a U-turn back toward a group that includes the suspect, and the suspect flees as the officers approach the group.