This Essay makes the case for "proportional mens rea," a proportionality-based approach to mens rea selection. Proportional mens rea would provide proportionality safeguards that are otherwise entirely lacking in substantive criminal law and,as a practical matter, unavailable in constitutional law. Creating implied mens rea requirements, where necessary to ensure proportional punishment, is not a judicial usurpation of a legislative function. Rather, it is to take seriously the role that courts play, under both constitutional and substantive criminal law, to ensure that punishment "fits" the crime. Moreover, proportional mens rea would represent a needed counterweight to prosecutorial behavior whereas current doctrine does not. Given that federal prosecutors do not seek to charge morally blameless people, mens rea doctrine aimed only at protecting moral blamelessness from punishment will largely be redundant of prosecutorial discretion. Proportionality of punishment, however, is a concern that federal prosecutors—bound by long standing Executive Branch mandates to seek the maximum supportable penalty in every case and oppose lenient exercises of judicial sentencing discretion—routinely ignore. Judicial mens rea selection, therefore, has a substantial contribution to make to the achievement of proportionality of punishment.