An ex post facto law is a law passed after the occurrence of an act which retrospectively changes the juridical consequences of this act. Historically, a bill of attainder was a legislative act that imposed punishment on an individual or a group of people considered guilty, without a trial, though traditionally it was the function of a court to judge and impose punishment.
When the U.S. Constitution was adopted, many people understood ex post facto laws to include all retrospective laws both of a civil and a criminal nature. But in the early case of Calder v. Bull, the Supreme Court decided that the term used in the Constitution applied only to penal and criminal statutes. Every law, which makes crime an act that was innocent when done, or which imposes a tougher punishment than was annexed to the crime when committed, is an ex post facto law prohibited by the Constitution.
Bills of attainder were widely in use through the 18th century in England, and were applied to British colonies in the U.S. as well. One of the impulses for the American Revolution was anger at the injust character of attainder. American dissatisfaction with attainder laws caused their further prohibition in the Constitution.
Both ex post facto and bill of attainder provisions have direct application in today’s legal system. In a lot of cases, an ex post facto law does not let a court find a person guilty of actions that were against a law prior to the law’s establishing. Moreover, this law may toughen crime punishments, and if the law was changed before the suspected came to trial, he or she might be subject to greater punishments than expected. However, applying tougher laws to the previously convicted in an ex post facto meaning is not allowed in the U.S. Sometimes a law can be changed ex post facto that makes a previously illegal behavior or event, legal. As a result, people who had been claimed guilty can resume normal lives.
In its turn, the Bill of Attainder clause in the Constitution protects citizens from the legislature that carries out judicial branch functions. It protects us from being claimed guilty by legislative act as opposed to a court or jury of peers.