The Fourth Amendment to the United States Constitution requires that law enforcement officers obtain a search warrant before searching a citizen or his or her property. There are certain situations, however, in which requiring the police to obtain a warrant before conducting a search would increase the likelihood that someone was injured or evidence was lost, to a degree that our Supreme Court has found to be unreasonable. It has therefore determined that in these special situations, the police may legally search a person and/or his or her property without first obtaining a search warrant. As of January 6, 2012, there are twelve situations in which the search warrant requirement does not apply, and the police are allowed to search a person, his or her vehicle, and sometimes even his or her home, without a warrant. The twelve exceptions to the search warrant requirement and an example of each are:
Anyone who watches television knows that the police must have probable cause in order to obtain a search warrant. What many do not know, is that there are instances in which probable cause justifies a warrantless search. The probable cause exception to the warrant requirement applies only to vehicles. It is a result of the 1925 case, Carroll v. U.S., where the United States Supreme Court held that a police officer could search a vehicle in transit, or one moving on public roads, if there was probable cause to believe that the vehicle was carrying evidence or contraband.  Evidence could be a dead body or stolen property. Contraband includes illegal drugs, weapons, or any item, which is illegal to own and/or transport. Probable cause, however, is subjective and its definition has been considered by many Courts. When applied to vehicle searches, it can be loosely defined as sufficient reason, based on the known facts of a specific situation, that the occupants are transporting evidence or contraband.
Example of a warrantless search based on probable cause
While conducting a traffic stop, a law enforcement officer smells marijuana. She searches the vehicle, discovers a bag of marijuana under the passenger’s seat, and arrests both occupants of the vehicle.
A consent search does not require a law enforcement officer to have probable cause. An officer is free to conduct a warrantless search of your person, vehicle, or home if he or she has your consent. If co-owners of a home are both present, they must both consent to the search. If only one co-owner is present, he or she may consent for the other in his or her absence.  For example, when a husband and wife own a home together and only wife is present when an officer asks to conduct a search of the home, wife may give consent for husband. There may be circumstances in which your home or vehicle may also be searched, in your absence, based on the consent of a third party. The party giving consent must be an adult who appears to have reasonable authority to consent to the search. You have the right to refuse to give consent to a search. You also have the right to limit the scope of the search for which you give consent. For example, when consenting to your home being searched, you may specify the home only, and not the attached garage.
Example of a warrantless search based on consent
While conducting a traffic stop, an officer begins to suspect that the teenage occupants of the vehicle have been smoking marijuana. He does not see or smell marijuana and does not have probable cause to search the vehicle. He asks the driver for consent to search the car, obtains it, and subsequently discovers a bag of marijuana. He arrests all of the vehicle’s occupants.
An inventory search is one, which is conducted in order to inventory and secure a person’s personal affects after he or she has been arrested or had their vehicle towed by police. Inventory searches are also conducted on an arrestee’s person in order to secure items he or she is carrying. Inventory searches were held to be reasonable by the U.S. Supreme Court in 1976 in the case of South Dakota v. Opperman. Here, the Court held that inventory searches conducted by police in order to secure personal property were a party of law enforcement’s ‘caretaking’ responsibilities, and therefore, not in violation of the Fourth Amendment. 
Example of a warrantless search for inventory purposes
During a routine traffic stop, an officer discovers that the driver of the vehicle has several outstanding warrants. He places the driver under arrest. While waiting for the tow truck, the officer searches the vehicle and makes a list of all items contained within both the passenger compartment and the trunk. The driver is then escorted to the police station where any items he is carrying in his pockets or in any bags or containers are itemized, removed from him, and placed in storage to be returned when he is released.
Incident to an arrest
When a person is arrested, law enforcement officers may search the area surrounding him or her to ensure he or she does not have access to any weapons, and that no evidence will be lost or destroyed.  For example, a small bag of cocaine, tossed by the arrestee before being arrested. This type of warrantless search was held to be reasonable by the United States Supreme Court in 1914, in the case of Weeks v. United States, and then limited to the “immediately” surrounding area in 1969, by Chimel v. California.  The area, which may be searched is referred to by law enforcement as the ‘grab area’. It is the area close enough to the arrestee that he or she could lunge and destroy evidence, or grab a weapon.
Example of a warrantless search incident to an arrest
After arresting a man at his front door, the police search his porch and the area of the yard surrounding the porch.
Under the Plain View Doctrine, an officer may lawfully seize evidence, which is in plain view from an area in which he or she is lawfully present. Arizona v. Hicks clarified the Plain View Doctrine in 1987, limiting ‘plain view’ to only those items that could be seen without moving anything.  Plain view was again limited in 1990, by Horton v. California, when the Court held that the office must immediately recognize the item in plain view as evidence without making any further intrusion, such as moving the item to locate a serial number.  The Horton ruling also expanded the Plain View Doctrine by eliminating the previous requirement that the evidence be inadvertently discovered.
Example of a warrantless Plain View seizure
While speaking to a woman in her living room, after receiving a report from her that her neighbor is harassing her, an officer notices a bag of marijuana lying on the coffee table beside a marijuana pipe. The officer seizes the bag and the pipe, and places the women under arrest.
If waiting for a search warrant would create a reasonable risk of harm to a law enforcement officer or other person, destruction or concealment of evidence, escape of a suspect, or any other consequence, which legitimately frustrates law enforcement, then an officer may conduct a search without obtaining one. 
Example of a warrantless Exigent Circumstances search
A woman calls 9-1-1 and tells the dispatcher that there is an intruder in her home. She hangs up before police arrive. A man answers the door for police and says that he is home alone. The man is unable to produce identification. The police detain the man while they search the home for the woman.
Stop and Frisk or Terry Stop
A Stop and Frisk, commonly known as a Terry Stop, may be conducted by an officer in order to ensure his or her safety while questioning a person whom he or she has reasonable suspicion to believe is engaged in criminal activity. A Terry Stop is a brief detention, lasting only a few minutes, where an officer “pats down”, or frisks, a person before questioning him or her. The term “Terry Stop” is derived from the case Terry v. Ohio, which ruled this type of warrantless search lawful in 1968. 
Example of a warrantless Stop and Frisk, or Terry Stop
After witnessing two women exchange, what appear to be money and a small bag, a police officer stops one of the women and pats her down before questioning her regarding the exchange.
When in ‘hot pursuit’ of a felony suspect, an officer may enter any building, which he or she reasonably believes the suspect entered, in order to arrest the suspect. Because there is a need to act quickly in order to ensure that the suspect does not get away and evidence is not lost, a warrantless entry, during hot pursuit, to make an arrest was found to be reasonable by the Court in 1976, in the case of United States v. Santana. The Santana Court held that a defendant could not defeat a lawful arrest, set in motion in a public place, by retreating into a private one. 
Example of a warrantless Hot Pursuit entry
A police officer is chasing a suspect, thought to have robbed a convenience store at gun point, down a sidewalk, when the suspect turns and runs into a private residence. The officer follows the suspect into the residence and places him under arrest
In 1924, the United States Supreme Court held, in Hester v. United States, that because no expectation of privacy exists in an open field, the Fourth Amendment protections against warrantless searches of our persons, papers, houses, and effects, did not extend to open fields.  This is known as the Open Field Doctrine. The Doctrine was upheld in 1984 in Oliver v. United States, when the Court ruled that open fields “do not provide the setting for those intimate activities that the Amendment is intended to shelter from government interference or surveillance.” 
Example of a warrantless Open Field search
Two police officers observe several automobiles and automobile parts in an open field adjacent to the suspected owner of a chop shop. The officers seize the vehicles and vehicle parts.
Abandoned property may be a vehicle, personal property such as a purse or wallet, or a hotel room. By abandoning property, a person gives up his or her rights in the property, leaving law enforcement free to search it without obtaining a warrant. In order to determine if property has been abandon, the Court uses the “reasonable person” and “totality of circumstances” tests.  What this means, is that if a reasonable person would conclude, based on the totality of the circumstances, or all of the evidence, that the property had been abandoned, than it is considered to be abandoned and the police may search it without obtaining a warrant.
Example of a warrantless Abandoned Property search
A law enforcement officer responds to a report of an abandoned car parked along a country road. The officer tags the car with a warning for the owner to move it within 48 hours. He returns 72 hours later and finds the car where he left it with no sign that the owner has returned. The officer searches and impounds the car as abandoned property.
United States Border
Under Title 19 § 1581 of the United States Code, a law enforcement officer may search any person and/or his or her vehicle at the border where he or she is crossing into the United States without probable cause or even reasonable suspicion. 
Example of a warrantless United States Border search
Agents at the U.S./Mexican border ask two men to pull over and exit the vehicle. They then conduct a search of the vehicle and pat down both men before allowing them to cross into the United States.
Business, which are regulated by U.S. law, such as power plants, nursing homes, and restaurants, may be subject to warrantless administrative searches and inspections, meant to insure that the business is complying with certain administrative laws. The Courts have held that because these inspections are done for administrative, and not criminal investigatory purposes, they are not subject to the warrant requirement. 
Example of a warrantless administrative search
Pursuant to the Federal Mine Safety and Health Act, government inspectors arrive, without warning, to conduct a safety inspection of a commercial mine.
 Carroll v. U.S., 267 U.S. 132, U.S. (1925). http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&vol=267&invol=132
 South Dakota v. Opperman, 428 U. S. 364, U.S. (U.S. 1976). http://supreme.justia.com/us/428/364/case.html
 Weeks v. United States, 232 U. S. 383, (U.S. 1914 ). http://supreme.justia.com/us/232/383/case.html
 Chimel v. California, 395 U.S. 752 (U.S. 1969). http://supreme.justia.com/us/395/752/case.html
 Arizona v. Hicks, 480 U.S. 321, (U.S. 1987). http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&vol=480&invol=321
 Horton v. California, 496 U.S. 128, (U.S. 1990). http://supreme.justia.com/us/496/128/
 Terry v. Ohio, 392 US 1, (U.S. 1968). http://www.law.cornell.edu/supct/html/historics/USSC_CR_0392_0001_ZS.html
 United States v. Santana, 427 U.S. 38, (U.S. 1976). http://caselaw.lp.findlaw.com/scripts/getcase.pl?court=US&vol=427&invol=38
 Hester v. United States, 265 U.S. 57, (U.S. 1924). http://supreme.justia.com/us/265/57/case.html
 Oliver v. Unites States, http://www.law.cornell.edu/supct/html/historics/USSC_CR_0466_0170_ZS.html
 Search of Abandon Property: Fourth Amendment Considerations. http://findarticles.com/p/articles/mi_m2194/is_n5_v63/ai_16045564/
 Title 19 § 1581 of the United States Code. http://www.law.cornell.edu/uscode/html/uscode19/usc_sec_19_00001581—-000-.html
 In re Strouse, 23 Fed. Cas. 261 (D. Nev. 1871); In re Meador, 16 Cas. 1294, 1299 (N.D. Ga. 1869)