Police need a warrant, or an exception, before they can search.
The Fourth Amendment protects all citizens from unreasonable searches and seizures. The Supreme Court tells us that any search that is conducted without a warrant is, per se, unconstitutional, and therefore, illegal- unless an exception to the warrant requirement applies. Certainly, there are a variety of exceptions to the warrant requirement that the courts have created over the years. For example, if an individual consents to the search of his home or his car or his property, the police are entitled to conduct that search without a warrant. If evidence is seen in plain view, the police don't need a search warrant to take it. If there are exigent circumstances, a burning building or someone screaming, police typically don't need a warrant to further pursue and investigate those circumstances. Likewise, if police possess probable cause to believe that a car contains contraband or evidence of a crime, they don't have to separately get a warrant before searching that vehicle.