Procedural default occurs when a State court refuses to hear a Federal law claim because the claim was not brought properly. Only an extraordinary justification will permit consideration of the defaulted claim in Federal habeas corpus proceedings. Two kinds of justification are a showing of cause and prejudice or a showing that the defendant has suffered a fundamental miscarriage of justice. The article focuses on the Supreme Court's treatment of cases in which a fundamental miscarriage of justice has been asserted, with emphasis on the Court's concern for the issue of actual innocence in habeas corpus cases. The article concludes that the Supreme Court must eventually define miscarriage of justice in death penalty sentencing. 205 footnotes.