The 4th Amendment deals with unwarranted searches and seizures and lays out the basic limitations for how and when officers of the law can search your person or belongings. However, many people think the 4th Amendment applies to their situation without really understanding the specifics. The 4th Amendment says:
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
One important thing to note here are that this only applies to those acting on behalf of the government. While evidence may have to be excluded if a nongovernment party conducts the search, it's not a violation of the 4th Amendment.
Requirements for a search warrant
The most important part of obtaining a search warrant is showing probable cause. The official trying to get the warrant must show the judge that there is a reasonable belief that the search will uncover evidence of illegal activity or evidence that will have a significant impact on the outcome of the case. If the police have witness testimony that a person was party to a crime and the accused doesn't have a viable alibi, this could be enough to get a search warrant, for example.
Those requesting the warrant also need to know what they want to search and what they are looking for. For instance, a search warrant for a drug crime may specify that the officers are looking for drugs, paraphernalia and weapons.
There are some situations in which an officer can conduct a legal search without a warrant. If you are arrested in a public place for a felony, the officer can conduct a warrantless search as long as he has probable cause. Any time a person is arrested, the officer can search the person, the person's clothing and any areas in the person's reach without a warrant. If you are pulled over for a traffic violation, the officer can legally search your vehicle's interior and glove compartment. Lastly, a warrantless search may be legal in "exigent" or emergency circumstances that present an immediate danger, such as shots being fired or the officer hearing screams for help from inside a home.