The current project aims to increase understanding regarding the utility of existing TBI classification systems/criteria. This, in part, will be accomplished by exploring potential differences among those who meet criteria for mTBI between systems, and clarifying whether one or more of the systems misrepresents the functional burden of mTBI. Towards this end, members of the Technical Expert Panel (TEP; providers and those with lived experience) will assist in identifying and ranking, in terms of importance, psychosocial and environmental factors (PEFs; e.g., demographic characteristics, military characteristics, clinical factors [e.g., previous history of TBI], SDOH), expected to impact outcomes following mTBI. Study aims will be accomplished using Delphi methodology and secondary data analyses with variables available within a harmonized longitudinal data set of those with and without a TBI.