The jury trial, which is a hallmark of the Anglo-American adversary system, requires close attention to the evidence that it is permissible for the lay jurors to hear. No evidentiary issue has proved more contentious than the admissibility of witnesses’, especially defendants’, prior criminal history because of concern that the lay jurors might prejudicially infer present guilt from past criminality. This article explains the complex evidentiary rules for admitting criminal history to prove guilt and to impeach witness credibility. It suggests that inquisitorial trial procedure, which historically has been unconcerned that judges know about the defendant’s prior criminal history while they are determining present guilt may have to restrict admissibility of such evidence as lay juries become more common.