Almost fifteen years have passed since the U.S. Congress initiated the last wave of statutes that required federal agencies to promulgate rules to protect the public health and safety and that empowered the appellate courts to review those rules. Quite early in the evolution of this new area of administrative law, the courts invoked the partnership metaphor to illuminate the interaction between the rulemaking agencies and the reviewing courts.' Yet experience, not metaphors, defines institutional relationships. To date, judicial review of complex technical rulemaking has resulted in few clear-cut guidelines on the extent to which courts should defer to agency discretion in promulgating rules. And the potential for judicial review has had an important impact on the way that agencies go about promulgating technically complex rules. At the heart of this crucial question of discretion is the allocation of institutional power.