Both searches and seizures are simply an intrusion on a person’s liberty or liberty interest. The degree of intrusion depends on the type of search or seizure, and the Fourth Amendment prohibits unreasonable ones. Thus, in order for a police officer to legally conduct a search or seizure, it must be reasonable. Because certain intrusions are more severe than others, each type of search or seizure must meet certain requirements to be deemed reasonable under the Fourth Amendment. Intuitively, when the intrusion is more severe the officer should be more certain that it is warranted. Thus, it may be helpful to think of each reasonableness requirement as a level of certainty. For example, the most well-known requirement, probable cause, has the level of certainty built right in the name. If George told Sara “I think it’s probable Jake has Marijuana in his backpack,” Sara would understand that George is at least somewhat certain Jake is carrying marijuana. Another well-known degree of certainty is reasonable doubt. This is a higher degree of certainty because George could harbor no reasonable doubt that Jake was carrying Marijuana unless he was very certain he was carrying.
Thus, the intrusions discussed below, and their corresponding reasonableness requirements, can be thought of as existing together on a sliding scale. As the intrusion becomes more severe, officers need to be more certain in their justification (for example, more certain that a suspect is carrying drugs, that they stole the jacket they are wearing, or that a weapon is hidden under their car seat). Conversely, if the intrusion is minimal enough, sometimes officers need no justification at all.
Seizures by law enforcement officers are generally broken down into three categories: consensual encounters, investigatory detentions, and arrests.  The degree of intrusion increases with each. To initiate each type of seizure, the officer must meet the appropriate reasonableness requirement discussed below. Searches are simpler. Most searches require a warrant, but there are several important exceptions that permit officers to make warrantless searches.  The procedures and requirements for getting a warrant are beyond the scope of this article. But, it suffices to say that a valid warrant will not be issued absent probable cause.
Lastly, note that the search and seizure rules below are presented at their most basic level. There are many nuances and, sometimes, outright exceptions to these rules. Persons who believe they have been the victim of an unconstitutional search or seizure should contact an experienced civil rights attorney to discuss their options.
II. Seizures A. The Three Types of Police Seizures and Their Corresponding Reasonableness Requirements1. Seizure Type: Arrest—Reasonableness Requirement: Probable Cause
The most intrusive seizure is an arrest, which occurs when a suspect is taken into custody. Where the arrest occurred, and for what offense, slightly alters the reasonableness requirement.
When an arrest for a felony occurs in public, officers need probable cause to lawfully seize the individual.  Probable cause has been described in various ways. The California Supreme Court has stated that probable cause to “arrest exists when the facts known to the arresting officer would lead a person of ordinary care and prudence to entertain an honest and strong suspicion that the person arrested is guilty of a crime.” The California Supreme Court has also defined probable cause “as having more evidence for than against; supported by evidence which inclines the mind to believe, but leaves some room for doubt.” Thus, the level of certainty required is relatively high; the officer must have a strong suspicion, not a mere hunch.
To make an arrest in public for a misdemeanor, the officer still needs probable cause, and in addition the crime must have occurred in the officer’s presence. A crime is committed in the officer’s presence merely if he or she is aware of it through any of their senses.
Lastly, if the arrest is made in a home, a warrant is required. Note that fleeing into a house after police attempt to make a valid arrest in public does not trigger the requirement to get a warrant.
2. Seizure Type: Investigatory Detention—Reasonableness Requirement: Reasonable Suspicion Plus Articulable Facts
An investigatory detention is less intrusive than an arrest because it is “limited in duration, scope and purpose.”  A person is sufficiently detained to trigger the reasonableness requirement when, “in view of all of the circumstances surrounding the incident, a reasonable person would have believed that he was not free to leave.”  It is generally said that in order to be reasonable, an investigatory detention requires the officer to have a reasonable suspicion, supported by articulable facts, that a crime has been committed by the detainee. The determination is made in light of the totality of the circumstances. 
I. Automobile Stops
Since an automobile stop is a seizure under the Fourth Amendment, officers need at least the level of certainty required to conduct an investigatory detention of a person on the street (that is, they require reasonable suspicion). Further, the officer can order all occupants out of the vehicle without any additional justification. The officer may also have a police dog sniff the vehicle without any additional justification. Finally, note that officers are permitted to make pre-textual stops. Meaning that if an officer has a hunch that a driver is carrying drugs, but lacks the reasonable suspicion necessary for an investigatory detention, the officer can pull the individual over for, say, a broken taillight.
3. Seizure Type: Consensual Encounter—Reasonableness Requirement: not Applicable
An officer merely approaching a fellow citizen and engaging in casual banter is not yet an investigatory detention (nor is it an arrest) and so requires no degree of certainty since no constitutional liberty is being infringed. If an officer commands the suspect to do something however (for example, to come over and talk to them), the interaction is much more likely to be an investigatory detention. 
B. The Fluctuating Nature of Encounters With Police
The nature of any encounter with the police can change rapidly. Consensual encounters can turn into investigatory detentions, detentions can become arrests, or officers may approach with the probable cause requirement already met and arrest someone immediately. Moreover, although the actual, subjective beliefs of the officer and suspect are not considered, the objective mental state of a reasonable person put in their position is. In other words, the type of encounter, and whether the reasonableness requirement is met, is based on what is going on inside each person’s head. Would a reasonable detainee believe they could not leave? Can the officer articulate facts to support their suspicion the suspect is carrying a weapon? The blurry lines separating the three types of encounters are a source of “endless” judicial debate. But, what is clear is that the reasonableness of any encounter is judged on a case-by-case basis. With the aforementioned considerations in mind, the following details can help clarify the type of encounter.
When determining whether a stop was a consensual encounter or an investigatory detention, recall that a person is sufficiently detained to trigger the reasonableness requirement when, “in view of all of the circumstances surrounding the incident, a reasonable person would have believed that he was not free to leave.” Some key factors indicating that a reasonable person would feel they cannot leave are: the presence of many officers (versus a few), whether the officer makes demands (versus requests), whether the officer uses an authoritative tone (versus a conversational one), whether the officer keeps (versus promptly returns) any documentation given to them (for example, a driver’s license), and whether the officer is blocking the path (versus standing off to the side).
When determining whether a stop was an investigatory detention or an arrest, recall that an investigatory detention is limited in duration, scope, and purpose. When a detention exceeds any of those bounds, it becomes an arrest requiring probable cause.  Put another way, the “detention must be temporary and last no longer than is necessary to effectuate the purpose of the stop. Similarly, the investigative methods employed should be the least intrusive means reasonably available to verify or dispel the officer’s suspicion in a short period of time.” Note that an officer may use force to detain a suspect even for an investigatory stop. For example, officers may place a detainee in handcuffs without turning the encounter into an arrest requiring probable cause.
The rules for when an officer may conduct a search are relatively simple. In general, officers need a warrant to search a person or a home, but not to search a vehicle. However, there are several important exceptions to the warrant requirement that greatly increase officers’ ability to conduct a search. The warrantless searches that relate to the detentions described in Part II are discussed below. Note that these are not exhaustive, there are additional permitted warrantless searches.
A. Searches of Persons and Vehicles Incident to Arrest
The police may search an arrestee for evidence and weapons contemporaneously with any valid arrest. The scope of this search is limited to the suspect and any areas he might reach. Further, officers may “inspect” any personal property discovered as part of the search. Recently, the United States Supreme Court held that a cell phone is not searchable incident to arrest, even though it may be on the arrestee’s person or within their reach.
Officers may search the passenger compartment of a vehicle incident to the arrest of a recent occupant only in one of two cases.  One, the arrestee is unsecured and may gain access to the vehicle (to, in theory, destroy evidence) or, two, the officer reasonably believes evidence of the crime the person was arrested for is in the vehicle. 
1. Searches of Persons and Vehicles Incident to a Terry Stop (Also Known as “Stop and Frisks”)
Under Terry, officers may conduct a limited search, such as a pat down for weapons, of the person detained, if they reasonably believe the suspect is armed and dangerous. Officers may also temporarily seize items, like luggage they reasonably suspect contains narcotics.
Automobile stops parallel investigatory detentions on the street, so the officer may search the driver as well as the passengers for weapons, upon reasonable suspicion that they are armed and dangerous. The officer may also search the passenger compartment of the vehicle. While the search is limited to weapons, if the officer discovers contraband other than weapons, they are certainly not required to ignore it.
B. Automobile Exception
The police may search the entirety of an automobile, not just the passenger compartment (as in a Terry stop), if they have independent probable cause to believe evidence of a crime is in the vehicle. This search can include the trunk, glove compartment, and any container that might contain the evidence sought.
III. Conclusion-Putting the Pieces Together
Figure 1: Summary of Basic Fourth Amendment Search and Seizure Rules
Figure 1 summarizes the rules discussed above. Keep in mind that none of the searches and seizures shown require a warrant. Meaning that the officer could develop the appropriate reasonableness requirement independently and on the spot. Thus, investigatory detentions, arrests, searches incident to each, and the independent automobile exception, taken together, provide powerful tools for law enforcement officers to search and seize.
Consider the following example. Suppose an officer has a hunch that a driver is carrying drugs, but at this point lacks the reasonable suspicion necessary to initiate an investigatory detention. The officer looks for a more obvious offense like a broken taillight or speeding, which they could likely find quite easily. Assuming they find something (like an expired registration tag) the officer now has the power to detain both the driver and any occupants. Once pulled over, the officer can order all the occupants out. If the officer reasonably believes any of the detainees are armed or dangerous, he or she can pat them down and search any area within their reach, including the passenger compartment of the car. Under the seat, the officer could find the drugs he or she initially suspected the occupants were carrying. Suppose instead that the officer had no reason to believe any of the detainees were armed. By virtue of the pre-textual stop, the officer now has a much more direct interaction with the occupants, and is able to use a fuller, more detailed range of senses (in other words, the officer can see into the car better and can now smell or hear evidence of drug use or possession). Thus, the officer can more easily develop the probable cause needed to make an arrest, which in turn would enable him or her to conduct a more thorough search incident to arrest.
This is just one example of how a fairly innocuous pre-textual traffic stop could lead to more intrusive searches and seizures. These intrusions are necessary in order for police officers to protect and serve the community, but that does not mean they are not misused or abused. The Constitution protects citizens from such abuses and anyone who believes they have been the victim of an unreasonable search or seizure should contact an experienced civil rights attorney to discuss their options.
 U.S. Const. amend. IV., available at Fourth Amendment.
 See discussion infra Part II.
 See Laurie L. Levenson, California Criminal Procedure § 3:3 (2014).
 E.g. , Wilson v. Superior Court, 34 Cal. 3d 777, 784 (Cal. 1983) (citing Florida v. Royer, 460 U.S. 491 (1983)).
 See infra note 38 and accompanying text.
 See discussion infra Part III.
 U.S. Const. amend. IV. (stating “no warrants shall issue, but upon probable cause”), available at Fourth Amendment.
 Cal. Penal Code § 834 (West 2015), available at California Legislative Information.
 United States v. Watson, 423 U.S. 411, 423 (1976); Cal. Penal Code § 836(a)(2) (West 2015), available at California Legislative Information.
 People v. Price, 1 Cal. 4th 324, 410 (Cal. 1991) (citing People v. Harris, 15 Cal. 3d 384, 389 (Cal. 1975)).
 People v. Ingle, 53 Cal. 2d 407, 413 (Cal. 1960) (citing, among others, People v. Nagle, 25 Cal. 2d 216, 222 (Cal. 1944)).
 People v. Hernandez, 47 Cal. 3d 315, 341 (Cal. 1988).
 Cal. Penal Code § 836(a)(1) (West 2015), available at California Legislative Information.
 In re Alonzo C., 87 Cal. App. 3d 707, 712 (Cal. Ct. App. 1978) (citing People v. Brown 45 Cal. 2d 640, 642 (Cal. 1955)).
 Payton v. New York, 445 U.S. 573, 576 (1980).
 United States v. Santana, 427 U.S. 38, 42 (1976).
 Wilson v. Superior Court, 34 Cal. 3d 777, 784 (Cal. 1983).
 Michigan v. Chesternut, 486 U.S. 567, 573 (1988) (citing United States v. Mendenhall, 446 U.S. 544, 554 (1980)).
 E.g. , Terry v. Ohio, 392 U.S. 1, 21 (1968). At the time, the decision in Terry was considered a landmark, as it was the first time the United States Supreme Court acknowledged a level of intrusion that did not require probable cause. Florida v. Royer 460 U.S. 491, 498 (1983) (“Prior to Terry . . . , any restraint on the person amounting to a seizure for the purposes of the Fourth Amendment was invalid unless justified by probable cause.”). The decision also established the constitutionality of limited searches conducted incident to such detentions, see discussion infra Part III.A.1. For these reasons, investigatory detentions are sometimes called “Terry stops” or a “stop and frisk.”
 People v. Souza, 9 Cal. 4th 224, 230 (Cal. 1994).
 See Michigan v. Long, 463 U.S. 1032, 1047–50 (1983) (discussing Court’s previous applications of Terry to suspects in vehicles in Pennsylvania v. Mimms, 434 U.S. 106 (1977), Adams v. Williams, 407 U.S. 143 (1972), and New York v. Belton, 453 U.S. 454 (1981)).
 Pennsylvania v. Mimms, 434 U.S. 106, 111 (1977).
 Illinois v. Cabelles, 543 U.S. 405, 409 (2005).
 Whren v. United States, 517 U.S. 806, 813 (1996).
 The implications of this are briefly explored in the example Part III.
 People v. Warren, 152 Cal. App. 3d 991, 996 (Cal. Ct. App. 1984).
 Honorable George Brunn, retired, California Judges Benchbook Search and Seizure 157 (2d ed. 2002).
 Levenson, supra note 3, at § 3:5.
 In re Tony C., 21 Cal. 3d 888, 892, 895 (Cal. 1978).
 See supra note 19 and accompanying text.
 Brunn, supra note 28, at 163–64.
 See supra note 18 and accompanying text.
 In re Carlos M., 220 Cal. App. 3d 372, 384 (Cal. Ct. App. 1990) (citing Dunaway v. New York, 442 U.S. 200, 212 (1979)) (“When the detention exceeds the boundaries of a permissible investigative stop, the detention becomes a de facto arrest requiring probable cause.”).
 Florida v. Royer, 460 U.S. 491, 501 (1983) (citing United States v. Brignoni-Ponce, 422 U.S. 873, 881–81 (1975)).
 People v. Johnson, 231 Cal. App. 3d 1, 12 (Cal. Ct. App. 1991).
 Johnson at 14 (citing In re Carlos M., 220 Cal. App. 3d 372, 385 (Cal. Ct. App. 1990)).
 Carroll v. United States, 267 U.S. 132, 153 (1925).
 Some of these exceptions are so powerful that they prompted the Supreme Court to remark that the term “exception” is a misnomer, given that searches incident to arrest, for example, occur much more often than their warrant-requiring counterparts. See Riley v. California, 134 S. Ct. 2473, 2482 (2014).
 For example, officers may search for and seize incriminating evidence in plain view, Brunn, supra note 28, at 325, or may search when consent is given, Brunn, supra note 28, at 370.
 For a convenient summary of the law governing searches incident to arrest, see Riley v. California, 134 S. Ct. 2473, 2482–84 (2014), which condenses the “trilogy” of precedential cases (Chimel v. California, 395 U.S. 752 (1969), United States v. Robinson, 414 U.S. 218 (1973), and Arizona v. Gant, 556 U.S. 332 (2009)) down to a few pages.
 Chimel v. California, 395 U.S. 752, 763–64 (1969).
 United States v. Robinson, 414 U.S. 218, 236 (1973).
 Riley v. California, 134 S. Ct. 2473, 2484 (2014) (citing United States v. Chadwick, 433 U.S. 1, 15 (1977)).
 Id. at 2493.
 Davis v. United States, 131 S. Ct. 2419, 2424–25 (2011) (citing Arizona v. Gant, 556 U.S. 332 (2009)).
 Id. (summarizing Arizona’s majority opinion).
 Terry v. Ohio, 392 U.S. 1, 27 (1968).
 United States v. Place, 462 U.S. 696, 706 (1983).
 See Michigan v. Long, 463 U.S. 1032, 1047–50 (1983) (discussing previous applications of Terry to suspects in vehicles in Pennsylvania v. Mimms, 434 U.S. 106 (1977), Adams v. Williams, 407 U.S. 143 (1972), and New York v. Belton, 453 U.S. 454 (1981)).
 Michigan v. Long at 1049.
 Id. at 1050.
 United States v. Ross, 456 U.S. 798, 809 (1982).
 Id. at 822–24.
 “As a practical matter, if an officer can’t find a traffic violation to stop a car, he isn’t trying very hard.” Orin Kerr, Can a Police Officer Lawfully Pull Over a Car For A Traffic Violation Based on an Erroneous Understanding of the Traffic Laws?, The Volokh Conspiracy (Dec. 21 2012), The Volokh Conspiracy.
Attorney David M. Helbraun at the Helbruan Firm in San Francisco emphasize litigation and conflict resolution in the areas of legal malpractice, civil rights & police misconduct and insurance & tort litigation.