Evidence of the developmental origins (by age 18) of mild mental retardation (MMR) in adults is discussed. Multiple sources of evidence are reviewed and evaluated. The availability and uses of evidence from the developmental period are cited in the context of evaluations for Social Security income (SSI) eligibility and death penalty appeals due to MMR. The use of records from educational, social services, medical, and juvenile justice settings are cited in the context of specific cases. The convergent validity principle is described as a means to make decisions in the typical situation of ambiguous information.