Abstract: Regularly invoked by the Supreme Court in diverse contexts, the maxim nemo iudex in sua causa - no man should be judge in his own case - is widely thought to capture a bedrock principle of natural justice and constitutionalism. I will argue that the nemo iudex principle is a misleading half-truth. Sometimes rulemakers in public law do and should design institutions to respect the value of impartiality that underlies the nemo iudex principle. In other cases, they do not and should not. In many settings, public law makes officials or institutions the judges of their own prerogatives, power, or legal authority. Officials or institutions may determine their own membership, award their own compensation, rule on the limits of their own jurisdiction, or adjudicate and punish violations of rules they themselves have created. I will attempt to identify the general conditions under which rule designers sensibly depart from, override, or qualify the nemo iudex principle. In some cases, there is no impartial official or institution in the picture, so that wherever decisionmaking authority is lodged, someone or other will have to be the judge in his own case. In other cases, even where it would be feasible to respect the principle, the costs of doing so will exceed the benefits. In general, this will be so when and because impartiality trades off against one or several competing considerations: the benefits of expertise, the value of institutional autonomy and independence, or the motivation and activity level of officials and institutions. The upshot is that it is never sufficient to argue that a proposed institution, or a proposed interpretation of ambiguous constitutional rules or practices, would violate the nemo iudex principle. One must go on to ask whether the conflict is avoidable or unavoidable, and, if it is avoidable, whether it would be good or bad overall to avoid it.