In the case of Beck v. Ohio, the U.S. Supreme Court ruled in 1964 that police must have probable cause to arrest a suspect without a warrant. The case involved Robert Beck, who was arrested by Cleveland police on suspicion of being involved in illegal gambling operations but without any specific evidence or probable cause for his arrest. Police searched him and found betting slips, which were used as evidence against him at trial where he was convicted for possessing gambling paraphernalia. Beck appealed his conviction arguing that his Fourth Amendment rights had been violated because there wasn't sufficient probable cause for his arrest and subsequent search. The Supreme Court agreed with Beck's argument stating that an officer's belief is not enough to justify an arrest; they need concrete reasons based on facts and circumstances within their knowledge reasonably trustworthy information to believe that a crime has been committed by the person they intend to arrest. The court reversed Beck’s conviction holding it unconstitutional due its violation of Fourth Amendment protections against unreasonable searches and seizures since no proper justification was provided for arresting him initially before conducting the search yielding incriminating evidence.

In the dissenting opinion for Beck v. Ohio, Justice Harlan argued that the arresting officer had reasonable grounds to arrest without a warrant due to his extensive experience and knowledge of criminal behavior. He believed that the majority's decision undermined law enforcement officers' ability to make judgments based on their professional expertise and experiences in similar situations. Furthermore, he contended that it was not necessary for an officer to have evidence sufficient enough for conviction before making an arrest; instead, they only needed probable cause or reasonable suspicion. The dissent also expressed concern about placing too much emphasis on hindsight analysis of police actions rather than considering what seemed reasonable at the time of action.