Immune-related adverse events (irAEs) associated with immune checkpoint inhibitor (ICI) therapy may vary substantially in their clinical presentation, including natural history, outcomes to treatment, and patterns. The application of clinical guidelines for irAE management can be challenging for practitioners due to a lack of common or consistently applied terminology. Furthermore, given the growing body of clinical experience and published data on irAEs, there is a greater appreciation for the heterogeneous natural histories, responses to treatment, and patterns of these toxicities, which is not currently reflected in irAE guidelines. Furthermore, there are no prospective trial data to inform the management of the distinct presentations of irAEs. Recognizing a need for uniform terminology for the natural history, response to treatment, and patterns of irAEs, the Society for Immunotherapy of Cancer (SITC) convened a consensus panel composed of leading international experts from academic medicine, industry, and regulatory agencies. Using a modified Delphi consensus process, the expert panel developed clinical definitions for irAE terminology used in the literature, encompassing terms related to irAE natural history (ie, re-emergent, chronic active, chronic inactive, delayed/late onset), response to treatment (ie, steroid unresponsive, steroid dependent), and patterns (ie, multisystem irAEs). SITC developed these definitions to support the adoption of a standardized vocabulary for irAEs, which will have implications for the uniform application of irAE clinical practice guidelines and to enable future irAE clinical trials., Competing Interests: Competing interests: JN—Consulting fees: AstraZeneca, Bristol Myers-Squibb, Roche/Genentech, Takeda, Pfizer, Daiichi Sankyo; Contracted research: AstraZeneca, Merck; Data Safety Monitoring Board: Daiichi Sankyo; NPI: 1710327721. LCC—Consulting fees: AbbVie; Contracted research: Bristol-Myers Squibb. CM—Nothing to Disclose. MBA—Consulting fees: Aveo, BMS, Eisai, Exelixis, Genentech, Iovance, Merck, Novartis, Pfizer, Roche, Pyxis, Werewolf, Asher Bio, Calithera, Idera, Agenus, Apexigen, Neoleukin, Adagene, AstraZeneca, Elpis, ScholarRock, Surface, ValoHealth, Sanofi, Fathom; Ownership Interest Less Than 5%: Werewolf, Pyxis, Elpis; NPI: 1306874201. JRB—Consulting fees: Amgen, BMS, Genentech/Roche, Eli Lilly, GlaxoSmithKline, Merck, Sanofi, Regeneron; Contracted research: AstraZeneca, BMS, Genentech/Roche, Merck, RAPT Therapeutics, Revolution Medicines; Data and Safety Monitoring Board/Committees: GlaxoSmithKline, Sanofi. SC—Consulting fees: Amgen, AstraZeneca, Ultrahuman, Oncovita; Fees for Non-CE Services: Amgen, AstraZeneca, BMS, Janssen, Merck, MSD, Novartis and Roche; Contracted Research: Amgen, Merck, Sanofi Aventis; As part of the Drug Development Department (DITEP) at Gustave Roussy, Principal/sub-Investigator of Clinical Trials Research Grants from, Non-financial support (drug supplied) from: Principal/sub-Investigator of Clinical Trials for: Abbvie, Adaptimmune, Adlai Nortye USA, Aduro Biotech, Agios Pharmaceuticals, Amgen, Argen-X Bvba, Astex Pharmaceuticals, Astra Zeneca Ab, Aveo, Basilea Pharmaceutica International, Bayer Healthcare Ag, Bbb Technologies Bv, Beigene, BicycleTx, Blueprint Medicines, Boehringer Ingelheim, Boston Pharmaceuticals, Bristol Myers Squibb, Ca, Celgene Corporation, Chugai Pharmaceutical Co, Clovis Oncology, Cullinan-Apollo, Curevac, Daiichi Sankyo, Debiopharm, Eisai, Eisai, Eli Lilly, Exelixis, Faron Pharmaceuticals, Forma Tharapeutics, Gamamabs, Genentech, Glaxosmithkline, H3 Biomedicine, Hoffmann La Roche Ag, Imcheck Therapeutics, Innate Pharma, Institut De Recherche Pierre Fabre, Iris Servier, Iteos Belgium SA, Janssen Cilag, Janssen Research Foundation, Kura Oncology, Kyowa Kirin Pharm. Dev, Lilly France, Loxo Oncology, Lytix Biopharma As, Medimmune, Menarini Ricerche, Merck Sharp & Dohme Chibret, Merrimack Pharmaceuticals, Merus, Millennium Pharmaceuticals, Molecular Partners Ag, Nanobiotix, Nektar Therapeutics, Novartis Pharma, Octimet Oncology Nv, Oncoethix, Oncopeptides, Orion Pharma, Ose Pharma, Pfizer, Pharma Mar, Pierre Fabre, Medicament, Roche, Sanofi Aventis, Seattle Genetics, Sotio A.S, Syros Pharmaceuticals, Taiho Pharma, Tesaro, Turning Point Therapeutics, XencorResearch Grants from Astrazeneca, BMS, Boehringer Ingelheim, GSK, INCA, Janssen Cilag, Merck, Novartis, Pfizer, Roche, Sanofi, Non-financial support (drug supplied) from Astrazeneca, Bayer, BMS, Boringher Ingelheim, GSK, Medimmune, Merck, NH TherAGuiX, Pfizer, Roche. DF—Salary: Palleon Pharmaceuticals; Ownership Interest Less Than 5 Percent: Palleon Pharmaceuticals. LMK—Salary: AstraZeneca; Ownership Interest Less Than 5 Percent: AstraZeneca. JM—Consulting fees: Novartis, Pfizer, Bristol-Myers Squibb, Takeda, Pharmacyclics, Audentes Pharmaceuticals, AstraZeneca, Deciphera, Ipsen, Boston, Biomedical, ImmunoCore, GlaxoSmithKline, Janssen, Myovant, Amgen, Boehringer, Cytokinetics. MCP— Employee of Merck Sharp & Dohme, a subsidiary of Merck & Co, Rahway, NJ, USA, and own stock (less than 5%) in Merck & Co, Rahway, NJ, US. JR—Nothing to Disclose. CR—Consulting fees: BMS, Roche, Pierre Fabre, Novartis, Sanofi, MSD, Pfizer, AstraZeneca. ES—NPI: 1467480160. MES-A—Consulting fees: Avenue Therapeutics ChemoCentryx, Celgene (All unrelated to the topic of interest. KS—Nothing to Disclose. MT—Nothing to Disclose. JW—Royalty: Less than 2000 dollars from a TIL growth patent by Moffitt Cancer Center; IP Rights: Named on a PD-1 biomarker patent by Biodesix, and on a CTLA4 biomarker patent by Moffitt Cancer Center, and a TIL growth patent by Moffitt Cancer Center; Consulting fees: Ad Boards for: BMS, GSK, Merck, Genentech, Astra Zeneca, Pfizer, Celldex, Biond, Evaxion, Regeneron, Alkermes, Novartis, Incyte, Moderna, Cytomx, Ultimovacs and Sellas; NPI: 1053348706; Ownership Interest Less Than 5 Percent: Biond, Neximmune MESA: Consultant fees: Pfizer, Eli Lilly, BMS. SITC Staff—CG, AK, NL, SMW: Nothing to Disclose., (© Author(s) (or their employer(s)) 2023. Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ.)