There is a burgeoning literature on second opinions in professional contexts, as when patients or clients seek advice from a second doctor or lawyer. My aim, by contrast, is to analyze second opinions as a central feature of public law. I will try to show that many institutional structures, rules and practices have been justified as mechanisms for requiring or permitting decisionmakers to obtain second opinions; examples include judicial review of statutes or of agency action, bicameralism, the separation of powers, and the law of legislative procedure. I attempt to identify the main costs and benefits of these second-opinion mechanisms, to identify conditions under which they prove more or less successful, and to consider how the lawmaking system might employ such mechanisms to greater effect. I claim, among other things, that Alexander Bickel’s justification of judicial review as a “sober second thought” is untenable, and that the Supreme Court should adopt a norm that two successive decisions, not merely one, are necessary to create binding law.