The Fourth Amendment to the Constitution, which states that no person shall be subject to “unreasonable” searches and seizures, is meant to protect U.S. citizens’ right to privacy. While it accomplishes this on the one hand, this Amendment also grants authority to the government and its appointed law enforcement agencies to conduct searches of citizens—and to take them into custody— without a warrant under “reasonable” circumstances.
Usually, a police officer must obtain a warrant before conducting a search or making an arrest. The officer must demonstrate to a judge that there is probable cause to suspect criminal wrongdoing before the judge will issue the warrant. However, there are circumstances in which an officer may conduct a search and even make an arrest without obtaining a warrant. Under these circumstances, police must justify their actions with reasonable suspicion.
Reasonable suspicion essentially means that, under the given circumstances, a reasonably objective officer would have reason to suspect criminal activity.
Reasonable suspicion and probable cause can be thought of like rungs in a ladder: without a warrant, a police officer must have reasonable suspicion in order to stop a person. If, for instance, the officer acts on reasonable suspicion to conduct a pat-down check for weapons and finds contraband, then the officer may have probable cause to initiate an arrest. In this case, reasonable suspicion allowed the officer to conduct a limited search which yielded evidence. That evidence then became probable cause to justify an arrest.