This study finds that, breaking with decades of slow improvement, U.S. reading and math scores on the National Assessment of Educational Progress (NAEP) and other assessments have seen historic declines since most states implemented national Common Core English and math curriculum standards six years ago. This descriptive analysis is designed to be understood by a general, non-technical readership. It primarily compares student achievement gains on the NAEP after implementation of Common Core to student achievement gains in the years preceding implementation of Common Core. Since test score results, by their nature, tend to "bounce" somewhat from one year to the next and gains are rarely perfectly smooth, a significant part of the analysis determines the average annual gain since implementation of Common Core and compares that to the average annual gain before implementation of Common Core. This report also includes a section addressing defenses by Common Core advocates denying responsibility for the poor results. The following state samples are included: (1) California; (2) Florida; (3) Georgia; (4) Illinois; (5) Kentucky; (6) Massachusetts; and (7) New York.