This paper presents an economic model of the harmful error rule in criminal appeals. We test the implications of the model against legal doctrines governing reversible and nonreversible error of criminal convictions and on a sample of more than 1,000 criminal defendants who appealed their convictions in the U.S. courts of appeals between 1996 and 1998. Among the more important theoretical and empirical findings of the paper are the following. Intentional prosecutor and judge errors are more likely to be found harmful and lead the appellate court to reverse the defendant's conviction than are inadvertent errors. Prosecutor errors are more likely to be forgiven than judge errors, in part because judge errors are likely to have greater influence on jurors. Errors are less likely to be harmful when defendants face a higher error-free probability of conviction. Finally, appellate courts are more likely to publish an opinion when they are reversing the lower court. Copyright 2001 by the University of Chicago.