Editors' Summary: It is a sign of the maturity of environmental law that criminal enforcement is coming of age. Although federal and state pollution control agencies have long had criminal enforcement powers, those powers were rarely used. One explanation may have been a reluctance to saddle businessmen confronted with complex and confusing regulatory demands with the stigma of criminal indictments. Anotherr may have been convern that it would be difficult to prove that violations were accompanied by the requisite mental state—most of the statutes limit criminal sanctions to "knowing" violations. Whatever the reasons for past reluctance to prosecute violators of pollution control statutes, that reluctance has disappeared, perhaps reflecting a decision that the statutes have been around long enough to be familiar to all in the regulated community, making knowing violations truly criminal in nature. The author describes the current federal and state criminal enforcement initiatives, surveys the criminal provisions of federal environmental statutes, analyzes key questions of proof under those provisions, and offers suggestions for attorneys representing persons under investigation.