What is probable cause? Probable cause is a legal standard generally requiring reasonable grounds to believe that a crime has been committed or that evidence of a crime may be found in a particular place. In the United States, probable cause can be relevant when police make an arrest, conduct certain searches, or request a search warrant. It is more than a mere suspicion but does not require the same level of proof needed to establish guilt at trial. Whether probable cause existed depends on the specific facts and circumstances of each situation, including the information available to law enforcement at the time of the search or arrest.