By the federal constitution Congress is expressly prohibited from passing any bill of attainder or ex post facto law (Art. I, § 9, ¶ 3) and the same prohibition is imposed on the states (Art. I, § 10, ¶ 1). A bill of attainder is a statute imposing criminal punishment without judicial trial for an act already committed without regard to whether it was by law criminal when done. The Parliament of Great Britain assumed the authority to pass such acts and to inflict the punishment thus imposed. In England the term bill of attainder was limited to statutes inflicting the death penalty and confiscation of property for acts already done, while similar statutes imposing a less degree of punishment were called bills of pains and penalties. But the general term bills of attainder is understood in this country as meaning a statutory imposition of criminal punishment for an act already committed and without any previous trial according to due process of law. It is plain that any such statute would be contrary to the fundamental conception of due process of law, which in criminal cases necessarily involves a judicial trial before imposition of punishment.

In a general sense the term ex post facto law might be applied to any law retrospective in its operations, but as used in the federal and state constitutions it is interpreted as applicable only to retrospective statutes providing for the punishment of crimes (Calder v. Bull). Other retrospective statutes may be valid unless they impair the obligation of contracts or deprive persons of property rights, as will be explained in a later chapter. (See below, § 272.)

An ex post facto law, then, within the meaning of the constitutional prohibitions, is a law which makes acts criminal which were not criminal when committed, or provides a more severe punishment for criminal acts already committed, or changes the rules of procedure so as to make it more difficult for the person accused of a crime already committed to defend in a prosecution for such crime (Kring v. Missouri, and Cummings v. Missouri). No matter how reprehensible or immoral an act may be when committed, if at that time it is not criminally punishable under the law then existing it cannot subsequently be made punishable by statute; and no matter how inadequate the punishment provided for an act already committed, the punishment cannot as to that particular act be made made more severe by statute; and no matter how technical or unreasonable the rules of evidence or the rules of procedure may be, by which one who has committed a crime may be enabled to escape punishment, such defects in the law cannot as to that particular criminal act be remedied by subsequent legislation. The whole theory of the criminal law is that no one shall be punished thereunder unless in a clear case and in strict compliance with the existing law; and on the whole such a policy is deemed to be promotive of the general public welfare, although in particular cases it may facilitate the escape from punishment of persons who plainly ought to be punished.