Note: This is an educational course, not legal advice.
Once there is a search, the general rule is that it needs a warrant: issued by a neutral judge, based on probable cause (a fair probability that evidence of a crime will be found, judged by the totality of the circumstances, Illinois v. Gates, 1983), and particularly describing what is to be searched. But the Court has recognized many exceptions, and in practice most searches are warrantless.
| Exception | Leading case | What it allows |
|---|---|---|
| Consent | Schneckloth v. Bustamonte (1973) | a search a person voluntarily agrees to; knowing of the right to refuse is not required |
| Plain view | Horton v. California (1990) | seizing evidence in plain view from a lawful vantage point, if its incriminating nature is immediately apparent |
| Search incident to arrest | Chimel v. California (1969) | searching the arrested person and the area within his immediate control; not a whole house. Cell phone data needs a warrant (Riley v. California (2014)) |
| Automobile exception | Carroll v. United States (1925) | searching a car with probable cause, because cars can be moved quickly |
| Exigent circumstances | Brigham City v. Stuart (2006) | emergencies: hot pursuit, preventing evidence destruction, helping someone in danger |
| Stop and frisk | Terry v. Ohio (1968) | a brief stop on reasonable suspicion of crime; a pat-down for weapons if the person may be armed and dangerous |
Why it works
The standards form a ladder. Reasonable suspicion (for a Terry stop) is less than probable cause (for an arrest, a warrant or the automobile exception), which is less than proof at trial. Each step up allows a greater intrusion. Exceptions have limits that track their reasons. A search incident to arrest protects officers and preserves evidence, so it reaches only what the arrested person could grab; in Riley v. California (2014) the Court held that the digital contents of a phone pose neither danger, so police generally need a warrant: in the Chief Justice's words, "get a warrant."
Key idea
Searches generally need a warrant on probable cause. Exceptions: consent, plain view, search incident to arrest (Chimel v. California (1969)), the automobile exception (Carroll v. United States (1925)), exigent circumstances, and stop and frisk on reasonable suspicion (Terry v. Ohio (1968)).
Worked example: A traffic stop
Police stop a car for speeding. An officer smells what he reasonably identifies as burning marijuana, in a state where it is illegal, and searches the trunk. Which exception, and is it satisfied?
- The odor gives probable cause to believe the car holds evidence of a crime.
- Under the automobile exception (Carroll), police may search a car, including its trunk and containers, with probable cause and no warrant.
Answer: Automobile exception; satisfied by probable cause.
Worked example: After an arrest at home
Officers arrest a suspect in his living room, then search every room of the house without a warrant. Valid?
- Chimel limits a search incident to arrest to the person and the area within his immediate control.
- Other rooms are beyond that area (a quick "protective sweep" for dangerous people is a separate, narrower rule).
Answer: No; searching the whole house exceeds Chimel.
Common mistake
Confusing probable cause with reasonable suspicion. Reasonable suspicion (for a Terry stop and frisk) is a lower standard than probable cause (for an arrest, a warrant or a car search). Check which intrusion is being justified.