To characterize retail food environments and identify areas with limited retail access, researchers, government programs, and community advocates have primarily used secondary retail food outlet data sources. This systematic review examines the evidence for validity reported for secondary retail food outlet data sources for characterizing retail food environments. A literature search was conducted through December 2012 to identify peer-reviewed published literature that compared secondary retail food outlet data sources to primary data sources (i.e., field observations) for accuracy of identifying the type and location of retail food outlets. Nineteen studies were included in the review. Researchers found that the evidence for validity reported varied by secondary data source examined, primary data-gathering approaches, retail food outlets examined, and geographic and sociodemographic characteristics. More than half of the studies (53%) did not report evidence for validity by type of food outlet examined and by a particular secondary data source.