BY THE 1930S, OBSERVATIONAL STUDIES HAD SUGgested a link between cigarette smoking and lung cancer and, by the 1950s, strong evidence for a causal role had emerged. However, comprehensive measures to reduce cigarette smoking in the United States were delayed until after the 1964 Surgeon General’s Report on Smoking and Health, in part due to the political influence of the tobacco industry. In 1977, the US Senate Select Committee on Health and Human Needs held hearings to explore the relationship between diet and chronic disease. At that time, the adverse effects of saturated fat on blood cholesterol levels were welldocumented, motivating the committee to recommend reducing this type of fat to 10% of calorie intake. The committee also recommended reducing total fat to 30% of calorie intake, despite concerns from scientists, such as the administrator of the US Department of Agriculture’s Agricultural Research Service, that “only limited knowledge exists concerning proper diets for humans.” For the next 20 years, reduction of fat consumption became the primary dietary goal of the US government and virtually all nutritionrelated professional health associations. As a result, fat consumption as a proportion of total energy decreased markedly from the 1970s to the 1990s. During this period, the prevalence of obesity and diabetes increased greatly. Indeed, some have argued that the focus on reducing all dietary fats has actually caused harm, by diverting attention away from more effective action and by encouraging substitution of processed carbohydrates for more healthful monounsaturated and polyunsaturated fats. As demonstrated by these 2 accounts, the decision of whether and how to act for many public health problems must be made amid evolving and incomplete scientific knowledge. Both premature and delayed governmental action can result in preventable injury, illness, death, and economic loss on a large scale. Today, excessive body weight threatens to overtake smoking as the leading cause of preventable death, and obesity comprises only one of many chronic diseases caused by poor diet quality. For this reason, numerous public health measures to improve diet and combat the obesity epidemic have been proposed, chief among them calorie labeling. Labeling requirements now exist or are being considered in a number of states and cities, and several federal bills on the issue have been introduced. However, many of these legislative acts have encountered intense opposition from the restaurant industry, which argues that the benefits of providing this information to consumers have not been proven. In this Commentary, we propose 4 criteria to assess whether governmental action to address a public health problem is warranted and apply these criteria to calorie labeling regulation for restaurants.