The August 25, 2011, issue of Nature includes a letter from Hermann Stamm, European Commission (EC) Joint Research Centre (JRC), Institute for Health and Consumer Protection (IHCP). In response to Andrew Maynard’s article entitled “Don’t define nanomaterials,” in the July 7, 2011, issue of Nature, Stamm argues that a definition of engineered nanomaterials for regulatory purposes is “urgently needed.” Maynard proposes that a “‘one size fits all’ definition of nanomaterials will fail to capture what is important for addressing risk,” and suggests that regulators use “a list of nine or ten attributes (including size and surface area) for which certain values trigger action.” Stamm, however, maintains that size is “the most appropriate parameter on which to base a broad definition.” Maynard concludes that adaptive regulations are necessary to respond to scientific evidence, while Stamm acknowledges that a definition “would need revision in line with fresh scientific evidence.”