Abstract: In an important but thus far unnoticed development, federal courts have created a new series of "default principles" for statutory interpretation, authorizing regulatory agencies, when statutes are unclear, (a) to exempt trivial risks from regulation and thus to develop a kind of common law of "acceptable risks," (b) to take account of substitute risks created by regulation, and thus to engage in "health-health" tradeoffs, (c) to consider whether compliance with regulation is feasible, (d) to take costs into account, and (e) to engage in cost-benefit balancing, and thus to develop a kind of common law of cost-benefit analysis. These cost-benefit default principles are both legitimate and salutary, because they give rationality and sense the benefit of the doubt. At the same time, they leave many open questions. They do not say whether agencies are required, and not merely permitted, to go in the direction they indicate; they do not indicate when agencies might reasonably reject the principles; and they do not say what, specifically, will be counted as an "acceptable" risk or a sensible form of cost-benefit analysis. Addressing the open questions, this essay urges that the principles should ordinarily be taken as mandatory, not merely permissive; that agencies may reject them in certain identifiable circumstances; and that steps should be taken toward quantitative analysis of the effects of regulation, designed to discipline the relevant inquiries. An understanding of these points should promote understanding of emerging "second generation" debates, involving not whether to adopt a presumption in favor of cost-benefit balancing, but when the presumption is rebutted, and what, in particular, cost-benefit analysis should be taken to entail.