With few exceptions, the US Endangered Species Act requires the designation of “critical habitat” for threatened and endangered species. This provides important protections, including a prohibition against adverse modification of designated habitat by federal agencies. Scientists with the US Fish and Wildlife Service develop critical habitat designations, which are then peer reviewed before being finalized by the secretary of the interior. We reviewed 169 peer reviews of 42 designations for 336 species finalized between 2002 and 2007 and determined whether there were changes in the area designated and whether those changes reflected the reviewers' advice. Thirty-four (81 %) of the 42 designations were reduced by an average of 43%. Eighty-five of the reviews recommended adding areas, and 19 recommended subtracting areas. Areas were added in response to only four reviews and subtracted in response to only nine. These results highlight the limitations of peer review of government decisions, which lack an arbiter to ensure that reviews are adequately considered.