The right of defendants to appeal against conviction or sentence is normally regarded as a fundamental human right. At present this right is laid down in numerous international treaties on human rights, as well as in the Statutes of international courts. The notion and purpose of appellate proceedings vary in national systems. Subject to a number of specifications and exceptions, in civil law countries, that is countries of Romano-Germanic legal tradition, these proceedings amount largely to a retrial by a court of appeal. In contrast, in most common law countries appellate proceedings do not lead to a retrial. Appeals courts, which do not have any jury, do not review facts, but decide on the basis of the trial record. In international criminal proceedings neither the common law system nor the civil law model have been upheld. Rather, a mixed system has been accepted, which is discussed in this chapter.