Hopt v. People was a United States Supreme Court case that addressed the issue of whether a state could impose criminal punishment for a crime that was committed before the state's law was enacted. The case involved a man named Hopt who was convicted of horse theft in Utah Territory in 1872. The crime of horse theft was not a crime in Utah Territory at the time of the offense, but it was made a crime in 1876. Hopt was convicted of the crime in 1877 and appealed his conviction to the Supreme Court. The Supreme Court held that the state of Utah could not impose criminal punishment for a crime that was committed before the law was enacted. The Court reasoned that the state could not retroactively punish someone for a crime that was not a crime at the time it was committed. The Court also noted that the state could not impose a punishment that was more severe than the punishment that was in place at the time the crime was committed. The Court held that the conviction of Hopt was unconstitutional and reversed the conviction.

Justice Field delivered the dissenting opinion in Hopt v. People, a case concerning whether or not an accused had been denied due process of law when he was convicted without being allowed to confront his accuser. Justice Field argued that the right to confrontation is fundamental and essential for a fair trial, and that it should be extended even if there are exceptions made by state laws. He further argued that allowing such exceptions would lead to arbitrary convictions based on hearsay evidence rather than direct testimony from witnesses who could be cross-examined under oath. In conclusion, Justice Field asserted that denying an accused this basic right violated their constitutional rights as guaranteed by the Fourteenth Amendment's Due Process Clause and thus overturned the conviction in question.