Aims: To compare the effectiveness and safety of clopidogrel, ticagrelor, and prasugrel in patients with ST-segment elevation myocardial infarction (STEMI) undergoing percutaneous coronary intervention (PCI)., Methods and Results: Nationwide, registry-based study of STEMI patients treated with primary PCI (2011-17) and subsequently with aspirin and a P2Y12 inhibitor. The effectiveness outcome was major adverse cardiovascular events (MACE) defined as a composite of recurrent myocardial infarction, repeat revascularization, stroke, or cardiovascular death at 12 months. The safety outcome was bleeding requiring hospitalization at 12 months. Multivariable logistic regression with average treatment effect modeling was used to calculate absolute and relative risks for outcomes standardized to the distributions of demographic characteristics of all included subjects. We included 10 832 patients; 1 697 were treated with clopidogrel, 7 508 with ticagrelor, and 1,627 with prasugrel. Median ages were 66, 63, and 59 years (P < 0.001). Standardized relative risks of MACE were 0.75 for ticagrelor vs. clopidogrel (95% confidence interval [CI], 0.64-0.83), 0.84 for prasugrel vs. clopidogrel (95% CI, 0.73-0.94), and 1.12 for prasugrel vs. ticagrelor (95% CI, 1.00-1.24). Standardized relative risks of bleeding were 0.77 for ticagrelor vs. clopidogrel (95% CI, 0.59-0.93), 0.89 for prasugrel vs. clopidogrel (95% CI, 0.64-1.15), and 1.17 for prasugrel vs. ticagrelor (95% CI, 0.89-1.45)., Conclusion: Ticagrelor and prasugrel were associated with lower risks of MACE after STEMI than clopidogrel, and ticagrelor was associated with a marginal reduction compared with prasugrel. The risk of bleeding was lower with ticagrelor compared with clopidogrel, but did not significantly differ between ticagrelor and prasugrel., Competing Interests: Conflict of interest: D.L.B. discloses the following relationships—Advisory Board: AngioWave, Bayer, Boehringer Ingelheim, Cardax, CellProthera, Cereno Scientific, Elsevier Practice Update Cardiology, High Enroll, Janssen, Level Ex, McKinsey, Medscape Cardiology, Merck, MyoKardia, NirvaMed, Novo Nordisk, PhaseBio, PLx Pharma, Regado Biosciences, Stasys; Board of Directors: AngioWave (stock options), Boston VA Research Institute, Bristol Myers Squibb (stock), DRS.LINQ (stock options), High Enroll (stock), Society of Cardiovascular Patient Care, TobeSoft; Chair: Inaugural Chair, American Heart Association Quality Oversight Committee; Consultant: Broadview Ventures; Data Monitoring Committees: Acesion Pharma, Assistance Publique-Hôpitaux de Paris, Baim Institute for Clinical Research (formerly Harvard Clinical Research Institute, for the PORTICO trial, funded by St. Jude Medical, now Abbott), Boston Scientific (Chair, PEITHO trial), Cleveland Clinic (including for the ExCEED trial, funded by Edwards), Contego Medical (Chair, PERFORMANCE 2), Duke Clinical Research Institute, Mayo Clinic, Mount Sinai School of Medicine (for the ENVISAGE trial, funded by Daiichi Sankyo; for the ABILITY-DM trial, funded by Concept Medical), Novartis, Population Health Research Institute; Rutgers University (for the NIH-funded MINT Trial); Honoraria: American College of Cardiology (Senior Associate Editor, Clinical Trials and News, ACC.org; Chair, ACC Accreditation Oversight Committee), Arnold and Porter law firm (work related to Sanofi/Bristol-Myers Squibb clopidogrel litigation), Baim Institute for Clinical Research (formerly Harvard Clinical Research Institute; RE-DUAL PCI clinical trial steering committee funded by Boehringer Ingelheim; AEGIS-II executive committee funded by CSL Behring), Belvoir Publications (Editor in Chief, Harvard Heart Letter), Canadian Medical and Surgical Knowledge Translation Research Group (clinical trial steering committees), Cowen and Company, Duke Clinical Research Institute (clinical trial steering committees, including for the PRONOUNCE trial, funded by Ferring Pharmaceuticals), HMP Global (Editor in Chief, Journal of Invasive Cardiology), Journal of the American College of Cardiology (Guest Editor; Associate Editor), K2P (Co-Chair, interdisciplinary curriculum), Level Ex, Medtelligence/ReachMD (CME steering committees), MJH Life Sciences, Oakstone CME (Course Director, Comprehensive Review of Interventional Cardiology), Piper Sandler, Population Health Research Institute (for the COMPASS operations committee, publications committee, steering committee, and USA national co-leader, funded by Bayer), Slack Publications (Chief Medical Editor, Cardiology Today’s Intervention), Society of Cardiovascular Patient Care (Secretary/Treasurer), WebMD (CME steering committees), Wiley (steering committee); Other: Clinical Cardiology (Deputy Editor), NCDR-ACTION Registry Steering Committee (Chair), VA CART Research and Publications Committee (Chair); Patent: Sotagliflozin (named on a patent for sotagliflozin assigned to Brigham and Women's Hospital who assigned to Lexicon; neither I nor Brigham and Women's Hospital receive any income from this patent.) Research Funding: Abbott, Acesion Pharma, Afimmune, Aker Biomarine, Amarin, Amgen, AstraZeneca, Bayer, Beren, Boehringer Ingelheim, Boston Scientific, Bristol-Myers Squibb, Cardax, CellProthera, Cereno Scientific, Chiesi, CSL Behring, Eisai, Ethicon, Faraday Pharmaceuticals, Ferring Pharmaceuticals, Forest Laboratories, Fractyl, Garmin, HLS Therapeutics, Idorsia, Ironwood, Ischemix, Janssen, Javelin, Lexicon, Lilly, Medtronic, Merck, Moderna, MyoKardia, NirvaMed, Novartis, Novo Nordisk, Owkin, Pfizer, PhaseBio, PLx Pharma, Recardio, Regeneron, Reid Hoffman Foundation, Roche, Sanofi, Stasys, Synaptic, The Medicines Company, 89Bio; Royalties: Elsevier (Editor, Braunwald’s Heart Disease); Site Co-Investigator: Abbott, Biotronik, Boston Scientific, CSI, Endotronix, St. Jude Medical (now Abbott), Philips, SpectraWAVE, Svelte, Vascular Solutions; Trustee: American College of Cardiology; Unfunded Research: FlowCo, Takeda. M.P. discloses the following relationships—Advisory Board: AstraZeneca, Janssen-Cilag; Speaker’s Fee: AstraZeneca, Bayer, Boehringer Ingelheim, Janssen-Cilag. The remaining authors declare no conflicts of interest., (© The Author(s) 2022. Published by Oxford University Press on behalf of the European Society of Cardiology. All rights reserved. For permissions, please email: journals.permissions@oup.com.)