In Gardner v. California, the U.S. Supreme Court ruled in 1968 that a defendant's constitutional rights were violated when a judge considered evidence not presented at trial during sentencing. The case involved Robert Gardner who was convicted of first-degree murder and sentenced to death by the Superior Court of Los Angeles County, California based on information from probation reports which were not disclosed to him or his counsel nor subject to cross-examination. The court held that due process requires full disclosure of any material used by the court in determining punishment so as to provide an opportunity for rebuttal or explanation by defense counsel. This ruling emphasized that defendants have a right under the Fourteenth Amendment’s Due Process Clause to confront and challenge all evidence used against them, even during sentencing proceedings.

In the dissenting opinion for Gardner v. California, Justice William O. Douglas argued that the defendant's constitutional rights were violated because he was not given an opportunity to confront or cross-examine witnesses whose statements were used against him in a pre-sentence report. The court relied on this report to impose capital punishment, but it contained hearsay evidence and personal opinions from probation officers which could have been biased or inaccurate. According to Douglas, due process requires that defendants be allowed to challenge such information before they are sentenced based on it. He also criticized the majority for failing to establish clear guidelines about what kind of information can be included in these reports and how it should be evaluated by judges.