An Act (or Bill) of Attainder is a Parliamentary proceedure in which Parliament passes judicial sentence on an accused person as if it were a court of law. In essence Parliament acts in place of a judge and jury, with the Bill of Attainder supplanting a judicial verdict. The accused criminal is sentenced by statute of law rather than by a judiial decision.

In England the process of attainder was frequently used when a monarch - or Parliament itself - wanted to deal with political foes without the risk of a court trial. Attainder was used as early as the 14th century, when Parliament deposed the DeSpensers, favourites of Edward II.

Attainder was frequently used posthumously - to confiscate the property of those who had died in opposition to authority. Edward IV used attainder to acquire the property of many of his opponents following his victory at the Battle of Towton.

Its primary use, however, was in the years of political and religious upheaval surrounding the English Civil War. Attainder was often used instead of impeachment, presumably because attainder offered more scope for authority to impose penalties and gain property.

Some famous - and infamous - figures who had Bills of Attainder brought against them include, George, Lord Clarence, Cardinal Wolsey, Thomas Cromwell, Queen Katherine Howard, the Duke of Norfolk, the Earl of Surrey, Lord Danby, and the Duke of Monmouth.

Bills of Attainder, as with other legislation, could be initiated in either House of Parliament, though it was most common for them to be brought in the House of Lords. Attainder legislation had to pass through the same process as any other bill, and if agreed by both Houses after three readings, passed to the crown for royal assent. The accused person was allowed to present evidence, provide witnesses, and speak before both Houses during the proceedings.

The penalty for those against whom attainder was passed was frequently death. Even in those cases where the death penalty was not imposed, land and property could be confiscated, and titles revoked.