Purpose of Review <p>There is no single, standardized clinical instrument or biomarker available to categorize traumatic brain injury (TBI) severity that is deemed the official gold standard for classification. Instead, the current approach is to rely on several criteria to establish severity.</p> Recent Findings <p>There are a number of different classification systems in place each utilizing different indicators. Heterogeneity across classification systems introduces a challenge for research recruitment and obscures knowledge on severity-outcome links, thus limiting generalizability across investigations. There is no comprehensive review and summary of these criteria into one single system.</p> Summary <p>We provide a brief overview of TBI and a narrative review of available systems to classify TBI severity. Additionally, we propose minimum standardized criteria that should be reported in TBI research to differentiate between mild and moderate-severe injuries in the absence of one or more determining criteria. Ultimately, such criteria will help facilitate comparison and generalizability across studies that may lead to the development of severity-specific medical interventions.</p>