This systematic review examined 14 U.S. studies published since 2000 reporting prevalence estimates for intellectual disability (ID) or developmental disability (DD). Prevalence rates for children were between 11.0 and 13.4 per 1,000 for ID and between 45.8 and 69.9 per 1,000 for DD using data from 2010 or later. A 2015 Ohio study of adults yielded a prevalence estimate of 41.0 per 1,000 for DD. The only study of ID in adults and the only study incorporating DD Act definitions used the 1994/1995 NHIS-D yielding prevalence estimates for ID and/or DD of 38.2 per 1,000 for children birth to 5 years, 31.7 for children 6 to 18 years, and 7.9 per 1,000 for adults. Notable differences in prevalence estimates by age and operational definition have important implications for public policy and research. Serious surveillance gaps limit our understanding of service utilization rates, unmet needs, and health and other outcomes for adults with ID or DD.