Abstract: In many settings, decisionmakers seek second opinions, and are wise to do so. Sometimes decisionmakers do not seek second opinions when they should have, or seek them when they should not have. In yet other settings, legal rules require decisionmakers to seek second opinions before taking action. There is a burgeoning literature on second opinions in professional contexts, as when patients or clients seek advice from doctors or lawyers. My aim, by contrast, is to analyze second opinions as a technique for the design of lawmaking institutions. 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 second opinions, to identify the conditions under which second-opinion arguments prove more or less successful, and to consider how the lawmaking system might employ second-opinion mechanisms to greater effect. Part I provides an analytic taxonomy of second-opinion mechanisms and introduces some conceptual distinctions. Part II analyzes the main benefits and costs of second-opinion mechanisms, and then ties the benefits and costs together with some comparative statics, attempting to identify general conditions under which second-opinion mechanisms are desirable or undesirable. Part III applies the analysis to legislative structure and procedure, and to judicial stare decisis. I claim, among other things, that the Supreme Court should adopt a norm that two successive decisions, not merely one, are necessary to create binding law.