Probable cause is a concept which comes up most often in a legal context in the United States, where the Fourth Amendment includes a specific reference to probable cause. It refers to a reasonable belief that a crime has been committed and that a particular person may be involved or responsible, and is used as the grounds for a legal search or arrest.
The Fourth Amendment states that “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.” This has been interpreted to mean that people and their properties cannot be searched without probable cause, and that people can only be arrested or detained when the standards for probable cause have been satisfied.
In a case where law enforcement officers want to obtain a warrant to search a person or premises, or to perform an arrest, they must demonstrate to a judge that any reasonable person would agree that there are grounds for the warrant. Evidence such as the result of a criminal investigation, testimony from witnesses, and documented observations of law enforcement can be used as grounds for probable cause. The judge weighs this information to determine whether or not the warrant would be constitutional before issuing the warrant, and many deny the warrant if he or she feels that there is not enough information to justify it.
Police officers have also been granted the ability to conduct limited detentions and searches if they have a reasonable suspicion that a crime has been committed. For example, if a police officer sees a driver weaving on the road late at night, the officer can pull the driver over and request that the driver take a sobriety test. Likewise, if an officer perceives a threat to safety, someone can be searched without a warrant issued by a judge.
It is not necessarily required to prove beyond a reasonable doubt when satisfying the standards for probable cause. This comes later, when the case is brought to trial. Under the laws which offer due process and a fair trial, the prosecution in a case must demonstrate to the jury that there is no question or ambiguity in the case, and that if they convict, they are indeed convicting the right person. The standards for conviction are especially high in criminal cases, in recognition of the fact that an erroneous conviction could have a devastating impact on someone's life.