Seizure

• In constitutional terms, seizure occurs when police detain someone for purposes of criminally investigating or prosecuting him. Seizure may also occur when police seize evidence of criminal activity to use it in court.
  
  

Probable Cause

• Probable cause to seize evidence is established when police reasonably believe that the item in question is evidence of a crime. Probable cause to seize persons is more complex: a police officer must be able to show that, based on all facts to which he has access, the person seized either committed or was in the process of committing a crime. The judge decides what constitutes valid probable cause.
  
  

Exclusion

• Seizures without a valid warrant or probable cause are unconstitutional. Any evidence obtained as a result of unconstitutional seizure is subject to the exclusionary rule: the evidence may not be admitted against the defendant in a court of law.