In this episode Professor Thomas McDonnell, Professor of Law at the Elisabeth Haub School of Law explains the Mistake of Law defense.  Professor McDonnell begins by noting that ignorance of the law is never a defense to a crime, but then highlights three instances in which defendants can successfully avoid conviction based on his misunderstanding of the law with which they are charged;  (1) if the legislature creates a mistake of law defense (2) If the law changes ex post facto and (3) if denying the defense would be a violation of fundamental fairness.