Purpose: To update recommendations of the ASCO systemic therapy for hormone receptor (HR)-positive metastatic breast cancer (MBC) guideline., Methods: An Expert Panel conducted a systematic review to identify new, potentially practice-changing data., Results: Fifty-one articles met eligibility criteria and form the evidentiary basis for the recommendations., Recommendations: Alpelisib in combination with endocrine therapy (ET) should be offered to postmenopausal patients, and to male patients, with HR-positive, human epidermal growth factor receptor 2 (HER2)-negative, PIK3CA -mutated, ABC, or MBC following prior endocrine therapy with or without a cyclin-dependent kinase (CDK) 4/6 inhibitor. Clinicians should use next-generation sequencing in tumor tissue or cell-free DNA in plasma to detect PIK3CA mutations. If no mutation is found in cell-free DNA, testing in tumor tissue, if available, should be used as this will detect a small number of additional patients with PIK3CA mutations. There are insufficient data at present to recommend routine testing for ESR1 mutations to guide therapy for HR-positive, HER2-negative MBC. For BRCA1 or BRCA2 mutation carriers with metastatic HER2-negative breast cancer, olaparib or talazoparib should be offered in the 1st-line through 3rd-line setting. A nonsteroidal aromatase inhibitor (AI) and a CDK4/6 inhibitor should be offered to postmenopausal women with treatment-naïve HR-positive MBC. Fulvestrant and a CDK4/6 inhibitor should be offered to patients with progressive disease during treatment with AIs (or who develop a recurrence within 1 year of adjuvant AI therapy) with or without one line of prior chemotherapy for metastatic disease, or as first-line therapy. Treatment should be limited to those without prior exposure to CDK4/6 inhibitors in the metastatic setting.Additional information can be found at www.asco.org/breast-cancer-guidelines., Competing Interests: Reprint Requests: 2318 Mill Road, Suite 800, Alexandria, VA 22314; guidelines@asco.org Debra L. BartonResearch Funding: Merck Lesley J. FallowfieldHonoraria: Voluntis, Genomic Health, NanoString Technologies, Novartis, Pfizer, MSD, Novartis, AbbVie, Clovis OncologyConsulting or Advisory Role: Puma Biotechnology, Voluntis, AstraZeneca, Takeda, Genomic Health/Exact Sciences, Lilly, Seagen, RocheResearch Funding: Bristol Myers Squibb, Novartis, LillyTravel, Accommodations, Expenses: Genomic Health Stephen R. D. JohnstonHonoraria: Pfizer, Novartis, Eisai, AstraZeneca, Roche, LillyConsulting or Advisory Role: Lilly, Puma Biotechnology, Novartis, PfizerResearch Funding: Pfizer, Puma BiotechnologyExpert Testimony: Novartis Jennifer K. LittonHonoraria: UpToDateConsulting or Advisory Role: Pfizer, AstraZeneca, Medivation/Pfizer, Ayala PharmaceuticalsSpeakers' Bureau: Physicans' Education Resource, UpToDate, Med Learning Group, Medscape, Prime Oncology, Clinical Care Options, MedpageResearch Funding: Genentech, Pfizer, EMD Serono, AstraZeneca, Zenith Epigenetics, MerckPatents, Royalties, Other Intellectual Property: UptoDateTravel, Accommodations, Expenses: Physicans' Education Resource, Med Learning Group, Medscape, Clinical Care OptionsOther Relationship: Medivation/Pfizer Praveen VikasStock and Other Ownership Interests: Moderna Therapeutics, NovavaxResearch Funding: Sanofi Rachel L. YungResearch Funding: Novartis, Odonate Therapeutics, American Cancer Society/Pfizer Hope S. RugoHonoraria: Puma Biotechnology, MylanConsulting or Advisory Role: SamsungResearch Funding: Macrogenics, OBI Pharma, Eisai, Pfizer, Novartis, Lilly, Genentech, Merck, Immunomedics, Odonate Therapeutics, Daiichi Sankyo, Seattle Genetics, Sermonix Pharmaceuticals, AstraZenecaTravel, Accommodations, Expenses: Pfizer, Novartis, Macrogenics, Mylan, Daiichi Sankyo, AstraZeneca Spain, MerckOpen Payments Link: https://openpaymentsdata.cms.gov/summaryNo other potential conflicts of interest were reported.