The Fourth Amendment protects us from unreasonable search and seizures. Therefore, to search and seize evidence, reasonableness is required. In order to meet this requirement, a warrant is required unless one of six exceptions [search incident to arrest, plain view exception, consent, stop & frisk, automobile exception, hot pursuit, evanescent evidence and other emergencies] applies. A valid warrant is one that is issued by a neutral and detached magistrate, based upon probable cause and particularly describing the place to be searched and items to be seized.
Consider however, that in order to get to the point of requiring a warrant, one must first have a reasonable expectation of privacy in the place searched or item seized, because without this, there is no Fourth Amendment right to protect. There is no expectation of privacy in objects held out to the public such as handwriting, smells, sounds, etc.; or in areas “held out to the public”.
With regard to the search incident to a lawful arrest, any arrest is sufficient; the issuance of a traffic citation is not. United States v Robinson 414 US 218 (1973); Knowles v Iowa, 119 S Ct 484 (1999). The officer may search the person and areas within his “wingspan”, which follows him as he moves; Chimel v California, 395 US 752 (1969); and they may also protect themselves by sweeping the area beyond the wingspan if they believe accomplices may be present. Maryland v Buie, 494 US 325 (1990). The entire passenger compartment (versus trunk) of an automobile is within the occupant arrestee’s wingspan. New York v Belton, 453 US 454 (1981). Don’t forget also, that the search must be contemporaneous in time and place to the arrest according to Preston v United States, 376 US 364 (1964) and United States v Chadwick, 433 US 1 (1977).
The automobile exception is somewhat self-explanatory. If the police have probable cause to suspect that there is evidence of a crime in the automobile they may search it without getting a warrant because by the time they obtain a warrant, the evidence is likely to be gone due to the mobile nature of the automobile. Carroll v United States, 267 US 132 (1925). They may search the entire vehicle and all containers which might contain the object; United States v Ross, 456 US 798 (1982);and may even search the passengers according to Wyoming v Houghten, 119 S Ct 1297 (1999). Contemporaneousness is not a requirement and therefore, the vehicle may be towed and searched later. United States v Johns, 469 US 478 (1985).
The plain view exception is also self-explanatory. If the police are legitimately on the premises and discover evidence, fruits or instrumentalities of a crime, or contraband which they have probable cause to believe is such; and it is in plain view, then the police may seize these items without a warrant. Coolidge v New Hampshire, 403 US 443 (1971). Also, as to consent, if the consent to search is with apparent authority, and is voluntary and intelligent, the warrantless search is valid, even if the person consenting was not informed of the right to withhold consent. Schneckloth v Bustamonte, 412 US 218 (1973).
With the stop and frisk exception, the officer may stop a person without probable cause for arrest if there is an articulable and reasonable suspicion of criminal activity and frisk them if there is a reasonable belief that the person may be armed and presently dangerous. Terry v Ohio, 392 US 1 (1968); United States v Cortez, 449 US 411 (1981).
There is no general emergency exception, but the police may make a warrantless search and seizure for certain emergencies such as contaminated food or drugs, children in trouble, and burning fires; and if they are in hot pursuit of a fleeing felon under Warden v Hayden, 387 US 294 (1967); and may seize blood samples and fingernail scrapings if they are likely to disappear before a warrant can be obtained.
There are exceptions which fall under the category of administrative searches and seizures. Furthermore, searches at the border of the United States do not fall under the protection of the Fourth Amendment.