On July 1, 2009, the US state of California began enforcing a new menu-labelling law that requires chain restaurants to post on their menus the calories contained in their food items. Three other states – Oregon, Maine, and Massachusetts – have already passed similar regulations, as have 11 city and county governments. The trend has gathered strength quickly, mostly because of concern about the nation’s expanding waistlines. The next step is to deploy the practice nationally, and the Congress is about to debate such a law. Pressure for this type of move is coming from the obese, who represent more than a third of American adults, and their defenders. Overweight people often struggle to estimate the number of calories they consume when eating out and make mistakes when calculating how much food they should order. Proponents of menu labelling hope that knowing what is in their food may direct people to healthier items. In Los Angeles, for example, officials optimistically predict that menu labelling could prevent nearly 40% of the annual weight gain there. However, the effect of menu labelling on dietary choices remains unclear, and the regulations are too new to produce much evidence. Furthermore, some critics of the trend believe the public-health benefits of the new legislation are irrelevant. For them, the new regulations are welcome as part of a consumer’s wide-ranging right to know.