Reasonable suspicion is a legal standard of proof in United States law that is less than probable cause, which is the legal standard for arrests and warrants, but more than an "inchoate and unparticularized suspicion or hunch". It requires that police officers have specific and articulable facts, taken together with rational inferences from those facts, to suspect that an individual has been, is, or is about to be engaged in criminal activity. Reasonable suspicion is evaluated using the "reasonable person" or "reasonable officer" standard, in which a person in the same circumstances could reasonably suspect a person has been, is, or is about to be engaged in criminal activity.

Reasonable suspicion is a step before probable cause, and it does not provide grounds for arrest. However, if facts discovered during a detention provide probable cause that the suspect has committed a crime, an arrest can be made. Reasonable suspicion is used in determining the legality of a police officers decision to perform a search, and it is a standard lower than probable cause.

Reasonable suspicion is evaluated based on the specific circumstances of each case, and it is not a bright-line rule. It is a flexible standard that requires a common-sense conclusion about human behavior upon which practical people are entitled to rely. Reasonable suspicion can be used to justify a brief stop and questioning of an individual, but it does not allow for a full search. However, if police additionally have reasonable suspicion that a person is armed and dangerous, they may "frisk" the person for weapons, but not for contraband like drugs.

In summary, reasonable suspicion is a legal standard of proof in United States law that requires specific and articulable facts, taken together with rational inferences from those facts, to suspect that an individual has been, is, or is about to be engaged in criminal activity. It is a step before probable cause and is used to determine the legality of a police officers decision to perform a search. Reasonable suspicion is evaluated based on the specific circumstances of each case and is a flexible standard that requires a common-sense conclusion about human behavior.