Analysis of the data for the 1985-86 National Assessment of Educational Progress (NAEP)and its comparison with the results of previous NAEP assessments suggested a substantial decrease in performance in reading for 9- and 17-year-olds, but a slight improvement for 13-year-olds. NAEP staff examined a number of hypotheses about what might have caused such anomalous results. While some hypotheses, such as inaccuracies in sampling, scaling, and quality control, were ruled out beyond any reasonable doubt, results of the studies are inconclusive. Some changes in the assessment process are inevitable, however, and these changes are documented in this report. The possibility that one or a combination of such changes may have resulted in the declines in reading proficiency cannot be ruled out. The effect of such changes cannot be estimated from existing data.