Acquired resistance (AR) to programmed cell death protein 1/programmed death-ligand 1 [PD-(L)1] blockade is frequent in non-small-cell lung cancer (NSCLC), occurring in a majority of initial responders. Patients with AR may have unique properties of persistent antitumor immunity that could be re-harnessed by investigational immunotherapies. The absence of a consistent clinical definition of AR to PD-(L)1 blockade and lack of uniform criteria for ensuing enrollment in clinical trials remains a major barrier to progress; such clinical definitions have advanced biologic and therapeutic discovery. We examine the considerations and potential controversies in developing a patient-level definition of AR in NSCLC treated with PD-(L)1 blockade. Taking into account the specifics of NSCLC biology and corresponding treatment strategies, we propose a practical, clinical definition of AR to PD-(L)1 blockade for use in clinical reports and prospective clinical trials. Patients should meet the following criteria: received treatment that includes PD-(L)1 blockade; experienced objective response on PD-(L)1 blockade (inclusion of a subset of stable disease will require future investigation); have progressive disease occurring within 6 months of last anti-PD-(L)1 antibody treatment or rechallenge with anti-PD-(L)1 antibody in patients not exposed to anti-PD-(L)1 in 6 months., Competing Interests: Disclosure AJS reports consulting/advising role to J&J and Perceptive Advisors. SJA reports consulting/advising role from Bristol-Myers Squibb, Merck, CBMG, AstraZeneca, Memgen, RAPT, and Venn, Achilles Therapeutics, Celsius, Samyang Biopharma, Glaxo Smith Klein, Amgen, other from Bristol-Myers Squibb, Merck, Rapt, Achilles Therapeutics, Celsius, GlaxoSmithKline, Amgen, grants from Novartis; expenses from Bristol-Myers Squibb, Merck, CBMG, AstraZeneca, Memgen, RAPT, and Venn, Achilles Therapeutics, Celsius, Samyang Biopharma, Glaxo Smith Klein, Amgen, other from Bristol-Myers Squibb, Merck, Rapt, Achilles Therapeutics, Celsius, GlaxoSmithKline, Amgen, grants from Novartis. MMA reports grants and personal fees from Genentech, grants and personal fees from Bristol-Myers Squibb, personal fees from Merck, grants and personal fees from AstraZeneca, grants from Lilly, personal fees from Maverick, personal fees from Blueprint Medicine, personal fees from Syndax, personal fees from Ariad, personal fees from Nektar, personal fees from Gritstone, personal fees from ArcherDx, personal fees from Mirati, personal fees from NextCure, personal fees from EMD Serono. EF reports personal fees from AbbVie, personal fees from Amgen, personal fees from AstraZeneca, personal fees from Bayer, personal fees from Blue Print Medicines, personal fees from Boehringer Ingelheim, personal fees from Bristol-Myers Squibb, personal fees from Eli Lilly, personal fees from GSK, personal fees from Janssen, personal fees from Medscape, personal fees from Merck KGaA, personal fees from Merck Sharp & Dohme, personal fees from Novartis, personal fees from Roche, personal fees from PeerVoice, personal fees from Pfizer, personal fees from prIME Oncology, personal fees from Puma Biotechnology, personal fees from Roche, personal fees from Sanofi Genzyme, personal fees from Springer, personal fees from Takeda, personal fees from TouchIME, grants from Grant for Oncology Innovation (GOI), grants from Fundación Merck Salud, outside the submitted work; and Grífols: Independent Member of the Board. JG reports grants and personal fees from Bristol-Myers Squibb, grants and personal fees from Genentech/Roche, personal fees from Takeda, grants and personal fees from Blueprint, grants and personal fees from Loxo, personal fees from Oncorus, personal fees from Regeneron, personal fees from Pfizer, personal fees from Incyte, grants and personal fees from Novartis, grants and personal fees from Merck, personal fees from Agios, personal fees from Amgen, grants from Array, grants from Tesaro, grants from Moderna, grants from Adaptimmune, grants from Alexo, personal fees from Ironwood Pharmaceuticals, personal fees from Gilead, personal fees from EMD Serono, personal fees from AstraZeneca, personal fees from Mirati. SNG reports personal fees and other from Bristol Myers Squibb, personal fees from Nektar, other from Genentech, other from Ariad/Takeda, other from Iovance Biotherapeutics, during the conduct of the study. FSH reports grants, personal fees and other from Bristol-Myers Squibb, personal fees from Merck, personal fees from EMD Serono, grants, personal fees and other from Novartis, personal fees from Surface, personal fees from Compass Therapeutics, personal fees from Apricity, personal fees from Aduro, personal fees from Sanofi, personal fees from Pionyr, personal fees from 7 Hills Pharma, personal fees from Verastem, personal fees from Torque, personal fees from Rheos, personal fees from Kairos, personal fees from Bicara, from Psioxus Therapeutics, other from Pieris Pharmaceutical, from Zumutor, from Corner Therapeutics, personal fees from Eisai, personal fees from Checkpoint Therapeutics, outside the submitted work; In addition, FSH has a patent Methods for Treating MICA-Related Disorders (#20100111973) with royalties paid, a patent Tumor antigens and uses thereof (#7250291) issued, a patent Angiopoiten-2 Biomarkers Predictive of Anti-immune checkpoint response (#20170248603) pending, a patent Compositions and Methods for Identification, Assessment, Prevention, and Treatment of Melanoma using PD-L1 Isoforms (#20160340407) pending, a patent Therapeutic peptides (#20160046716) pending, a patent Therapeutic Peptides (#20140004112) pending, a patent Therapeutic Peptides (#20170022275) pending, a patent Therapeutic Peptides (#20170008962) pending, a patent Therapeutic Peptides Patent number: 9402905 issued, a patent Methods of using pembrolizumab and trebananib pending, a patent Vaccine compositions and methods for restoring NKG2D pathway function against cancers Patent number: 10279021 issued, a patent Antibodies that bind to MHC class I polypeptide-related sequence A Patent number: 10106611 issued, and a patent Anti- galectin antibody biomarkers predictive of anti-immune checkpoint and anti-angiogenesis responses. Publication number: 20170343552 pending. MLJ reports other from Achilles Therapeutics, grants and other from AstraZeneca, grants and other from Atreca, grants and other from Boehringer Ingelheim, grants and other from Calithera Biosciences, grants and other from EMD Serono, grants and other from Roche/Genentech, grants and other from GlaxoSmithKline, grants and other from Gritstone Oncology, grants and other from Guardant Health, grants and other from Incyte, grants and other from Janssen Research and Development, grants and other from Lilly, grants and other from Merck, grants and other from Mirati Therapeutics, grants and other from Novartis, grants and other from Pfizer, grants and other from Sanofi-Aventis, grants from AbbVie, grants from Acerta Pharma, grants from Adaptimmune, grants and other from Amgen, grants from Apexigen, grants from Array BioPharma, grants from BeiGene, grants from Checkpoint Therapeutics, grants from Corvus Pharmaceuticals, grants from CytomX, grants and other from Daiichi Sankyo, grants from Dynavax Technologies, grants from Genmab, grants from Genocea Biosciences, grants from Hengrui Therapeutics, grants from Immunocore, grants from Jounce Therapeutics, grants from Kadmon Pharmaceuticals, grants from Loxo Oncology, grants from Lycera, grants from Neovia Oncology, grants from OncoMed Pharmaceuticals, grants from Regeneron Pharmaceuticals, grants from Shattuck Labs, grants from Stem CentRx, grants from Syndax Pharmaceuticals, grants from Takeda Pharmaceuticals, grants from Tarveda, grants from University of Michigan, grants and other from WindMIL Therapeutics, grants from TCR2 Therapeutics, grants from Arcus Biosciences, grants and other from Ribon Therapeutics, grants from Seven and Eight Biopharmaceuticals, other from Bristol-Myers Squibb, personal fees from Astellas, personal fees from Otsuka, grants from BerGenBio, grants from Foundation Medicine, other from G1 Therapeutics, grants from Tmunity Therapeutics. NBL reports grants and personal fees for CME lectures from MSD and BMS. CML reports other from Cepheid, other from Syros, other from Foundation Medicine, other from Novartis, other from Astra Zeneca, other from Takeda, other from Amgen, other from Blueprints Medicine, other from Genentech, other from Eli Lilly/Loxo, other from Pfizer, other from Roche, other from Achilles, from null, outside the submitted work. TM reports personal fees from AbbVie, Inc., personal fees and other from ACEA Pharma, personal fees and other from Alpha Biopharma Co. Ltd., personal fees and other from Amgen, personal fees and other from Amoy Diagnostics Co. Ltd., grants, personal fees and other from AstraZeneca, personal fees and other from BeiGene, personal fees and other from Boehringer Ingelheim, grants, personal fees and other from Bristol-Myers Squibb, personal fees and other from Blueprint Medicines Corporation, personal fees and other from CStone Pharmaceuticals, personal fees and other from Daiichi Sankyo, personal fees and other from Eisai, personal fees and other from Fishawack Facilitate Ltd., other from geneDecode, personal fees and other from Gritstone Oncology Inc., personal fees and other from Guardant Health, personal fees and other from Hengrui Therapeutics, personal fees and other from Ignyta Inc., personal fees and other from IQVIA, personal fees and other from Incyte Corporation, personal fees from InMed Medical Communication, personal fees and other from Janssen, personal fees and other from Lilly, personal fees and other from Loxo-Oncology, personal fees and other from Lunit, Inc., personal fees from MD Health (Brazil), personal fees from Medscape/WebMD, grants, personal fees and other from Merck Serono, grants, personal fees and other from Merck Sharp & Dohme, personal fees and other from Mirati Therapeutics Inc., personal fees from MoreHealth, grants, personal fees and other from Novartis, personal fees and other from OrigiMed, personal fees from PeerVoice, personal fees from Physicians' Education Resource, personal fees from P. Permanyer SL, grants, personal fees and other from Pfizer, Inc, personal fees from prIME Oncology, personal fees and other from Puma Technology Inc, personal fees from Research to Practice, grants, personal fees and other from Roche, personal fees and other from Sanofi-Aventis R&D, personal fees and other from Takeda, personal fees from Touch Medical Media, other from Virtus Medical Group, personal fees and other from Yuhan Corporation, other from AstraZeneca PLC, other from Hutchison Chi-Med, other from Sanomics Ltd., grants from Clovis Oncology, grants from SFJ Pharmaceuticals, grants from XCovery, outside the submitted work. MP reports grants, personal fees and non-financial support from Roche, grants, personal fees and non-financial support from AstraZeneca, personal fees from ELI LILLY, personal fees and non-financial support from Pfizer, grants, personal fees and non-financial support from Boehringer Ingelheim, personal fees and non-financial support from B Bristol Myers Squibb, personal fees and non-financial support from Merck Sharp & Dohme, personal fees from Novartis, grants and personal fees from Takeda, personal fees and non-financial support from Chugai, personal fees from Amgen, personal fees from Illumina, personal fees from Gritstone, outside the submitted work. MR reports personal fees from Amgen, personal fees from AstraZeneca, personal fees from BMS, personal fees from Boehringer-Ingelheim, personal fees from Lilly, personal fees from Merck, personal fees from MSD, personal fees from Mirati, personal fees from Novartis, personal fees from Pfizer, personal fees from Roche, personal fees from Samsung, outside the submitted work. BS reports personal fees from Roche/Genentech, personal fees from Novartis, personal fees from AstraZeneca, personal fees from Amgen, personal fees from Eli Lilly, personal fees from Loxo Oncology, personal fees from Merck, personal fees from Bristol Myers Squibb, personal fees from PharmaMar, personal fees from Pfizer, outside the submitted work. J-CS reports other from AstraZeneca, other from AstraZeneca, during the conduct of the study; other from Gritstone Oncology, other from Daiichi Sankyo, other from Relay Therapeutics, outside the submitted work. DSWT reports grants and personal fees from Novartis, grants and personal fees from Bayer, personal fees from Boehringer Ingelheim, personal fees from Celgene, grants and personal fees from Astra Zeneca, personal fees from Eli Lilly, personal fees from LOXO, personal fees from Merck, grants and personal fees from Pfizer, personal fees from Roche, personal fees from Takeda, grants from GlaxoSmithKline, personal fees from Merrimack, outside the submitted work. SP reports personal fees from AbbVie, personal fees from Amgen, personal fees from AstraZeneca, personal fees from Bayer, personal fees from Biocartis, personal fees from Boehringer-Ingelheim, personal fees from Bistrol-Myers Squibb, personal fees from Clovis, personal fees from Daiichi Sankyo, personal fees from Debiopharm, personal fees from Eli Lilly, personal fees from F. Hoffmann-La Roche, personal fees from Foundation Medicine, personal fees from Illumina, personal fees from Janssen, personal fees from Merck Sharp and Dohme, personal fees from Merck Serono, personal fees from Merrimack, personal fees from Novartis, personal fees from Pharma Mar, personal fees from Pfizer, personal fees from Regeneron, personal fees from Sanofi, personal fees from Seattle Genetics and Takeda, personal fees from AstraZeneca, personal fees from Boehringer-Ingelheim, personal fees from Bristol-Myers Squibb, personal fees from Eli Lilly, personal fees from F. Hoffmann-La Roche, personal fees from Merck Sharp and Dohme, personal fees from Novartis, personal fees from Pfizer, personal fees from Takeda, non-financial support from Sponsored by Amgen, non-financial support from AstraZeneca, non-financial support from Boehringer-Ingelheim, non-financial support from Bristol-Meyers Squibb, non-financial support from Clovis, non-financial support from F. Hoffmann-La Roche, non-financial support from Illumina, non-financial support from Merck Sharp and Dohme, non-financial support from Merck Serono, non-financial support from Novartis, non-financial support from Pfizer, non-financial support from Sanofi, personal fees from Bioinvent, outside the submitted work. All fees to institution. MDH reports personal fees from Merck, personal fees from Genentech/Roche, grants, personal fees and non-financial support from BMS, personal fees and non-financial support from AstraZeneca, personal fees from Mirati, personal fees from Syndax, personal fees and other from Shattuck Labs, personal fees from Nektar, personal fees and other from Immunai, personal fees and other from Arcus, from Eli Lilly, during the conduct of the study; In addition, Dr. Hellmann has a patent A patent filed by Memorial Sloan Kettering related to the use of tumor mutational burden to predict response to immunotherapy (PCT/US2015/062208) licensed to PGDx., (Copyright © 2021 European Society for Medical Oncology. Published by Elsevier Ltd. All rights reserved.)