Even where there is a reasonable expectation of privacy, some searches may legally be conducted without a warrant.

Exceptions to the Search Warrant Requirement

Police may conduct a search without a warrant if certain exceptions apply:

• Consent: A person voluntarily agrees to let a police officer conduct a search, even if there is a reasonable expectation of privacy, or a roommate or spouse grants third party consent. You do not have to agree to a search without a warrant, however, police are not required to advise you that you may refuse consent for a search.
• Investigatory Stop: When there is reasonable suspicion that a crime is in progress or is about to occur, law enforcement officers may conduct certain searches without necessity of a search warrant.
• Search Incident to Arrest: The person was validly arrested and the search was reasonably conducted.
• Exigent Circumstances: These emergency circumstances apply to a variety of situations under which quick action is required. For example, if officers have reason to believe that evidence is being destroyed, or that someone is injured or otherwise in need of assistance, a search warrant may not be required.

An arrest warrant gives police permission to arrest the person named in the warrant. An arrest can be made without a warrant. An arrest without a warrant is lawful under certain circumstances, and unlawful absent other such as just cause; however, there are exceptions to those rules.

Generally speaking, the police can make an arrest without a warrant if:

• the police observe the person attempting or committing a crime, or
• a reliable informant provides reasonably sound information to the police regarding a felony crime and the person who committed that crime, and if time permits, the police verify the information, or if time does not allow, the police should be reasonably certain that the information is valid, or
• the time lost in obtaining a warrant would permit the perpetrator to escape or evidence to be lost, and
• the officer has probable cause for an arrest.

An arrest without a warrant can also be made where a valid search occurred which revealed evidence that lead to the arrest. As well, evidence observed, or in "plain view", can substantiate probable cause for an arrest, however, there are exceptions to what constitutes "plain view", and most of those exceptions regard the fourteenth amendment right to reasonable expectation of privacy. For example, a person's right to expect privacy is personal, so the person may be at any place and have a right to expect privacy so long as the "place" is not of such a nature that reasonable people would not expect privacy.

Another common means by which police make lawful arrests without warrants is in an emergency situation. If the situation is such that the suspected offender or evidence will be lost, and the police have probably cause to make an arrest, an arrest may be lawful even absent a warrant.

Note that warrants are presumably issued on evidence that established a connection between a known crime and the person named in the warrant. A judge (usually a magistrate judge) reasonably believed that the evidence presented to him established that a crime had been committed and that the person named in the warrant either committed the crime or was a party to the crime.