In the United States, 1 in 8 women will be diagnosed with invasive breast cancer in her lifetime. Breast cancer death rates are higher for women in the United States than any other cancer, followed by lung cancer ( National Cancer Institute, 2019 ). More than 70% of breast cancers are hormone receptor (HR)-positive and human epidermal growth factor receptor 2 (HER2)-negative, and of those patients, 40% have driver mutations in the gene phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha ( PIK3CA ) resulting in damaged phosphatidylinositol 3-kinase (PI3K) and uncontrolled cell growth (Mollon et al., 2018; Setiawan et al., 2009). These patients are initially treated with endocrine therapy, but resistance remains an issue. Inhibition of PI3K is a promising new approach to overcome resistance to endocrine therapy in breast cancer. Previous trials of PI3K inhibitors (pictilisib [GDC-0941], buparlisib [BMK120], and taselisib [GDC-0032]) in breast cancer have shown little efficacy secondary to toxicities due to their nonselectivity to PI3K subunits. Alpelisib is a selective inhibitor of PI3K for patients with HR-positive, HER2-negative, PIK3CA -mutated breast cancer who have progressed on endocrine therapy. This drug review will discuss the pharmacology of alpelisib, current data supporting its place in therapy, management of adverse events, and the clinical implications for advanced practitioners treating patients with HR-positive, HER2-negative breast cancer.