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IMPORTANCE: Measurement of delirium severity has been recognized as highly important for tracking prognosis, monitoring response to treatment, and estimating burden of care for patients both during and after hospitalization. Rather than simply rating delirium as present or absent, the ability to quantify its severity would enable development and monitoring of more effective treatment approaches for the condition. OBJECTIVES: To present a comprehensive review of delirium severity instruments, conduct a methodologic quality rating of the original validation study of the most commonly used instruments, and select a group of top-rated instruments. EVIDENCE REVIEW: This systematic review was conducted using literature from Embase, PsycINFO, PubMed, Web of Science, and Cumulative Index to Nursing and Allied Health Literature, from January 1, 1974, through March 31, 2017, with the key words delirium, severity, tests, measures, and intensity. Inclusion criteria were original articles assessing delirium severity and using a delirium-specific severity instrument. Final listings of articles were supplemented with hand searches of reference listings to ensure completeness. At least 2 reviewers independently completed each step of the review process: article selection, data extraction, and methodologic quality assessment of relevant articles using a validated rating scale. All discrepancies between raters were resolved by consensus. FINDINGS: Of 9409 articles identified, 228 underwent full text review, and we identified 42 different instruments of delirium severity. Eleven of the 42 tools were multidomain, delirium-specific instruments providing a quantitative rating of delirium severity; these instruments underwent a methodologic quality review. Applying prespecified criteria related to frequency of use, methodologic quality, construct or predictive validity, and broad domain coverage, an expert panel used an iterative modified Delphi process to select 6 final high-quality instruments meeting these criteria: the Confusion Assessment Method–Severity Score, Confusional State Examination, Delirium-O-Meter, Delirium Observation Scale, Delirium Rating Scale, and Memorial Delirium Assessment Scale. CONCLUSIONS AND RELEVANCE: The 6 instruments identified may enable accurate measurement of delirium severity to improve clinical care for patients with this condition. This work may stimulate increased usage and head-to-head comparison of these instruments.