Background. Measurement gaps continue to hamper fuller understanding of late-life disability trends and dynamics. This article reports findings that validate the self-reported components of the disability protocol to be used in the new National Health and Aging Trends Study. The protocol was designed to redress existing measures by attending to environmental aspects of disability, capturing a broader range of capacity to perform tasks and including participation restriction items.Methods. We undertook an in-person validation study to determine the reliability, validity, and initial measurement properties of the National Health and Aging Trends Study self-reported disability protocol (n = 326). A random subset (n = 111) was readministered the protocol within 2–4 weeks. The interview and reinterview included new self-reported measures of physical capacity, activity limitations, and participation restrictions, as well as established performance and cognitive tests. We calculated percent agreement and kappa between interviews for all self-reported items and summary measures. We also assessed the construct validity of summary measures through correlations with demographic characteristics, frailty, memory, and performance-based mobility and confirmed whether activity limitations and participation restrictions were distinct domains.Results. New items and derived summary measures demonstrate robustness over a short time period, with kappas for retained/recommended items in the .60–.80 range. The summary measures correlate as expected with age, sex, residential status, and established performance-based constructs. Two factors, representing activity limitations and participation restrictions, were confirmed.Conclusions. The National Health and Aging Trends Study protocol preserves the ability to examine more traditional measures of functioning while offering new insights into how activities are performed and preserving key conceptual distinctions.