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Cell cycle dysregulation is a hallmark of cancer that promotes eccessive cell division. Cyclin-dependent kinase 4 (CDK4) and cyclin-dependent kinase 6 (CDK6) are key molecules in the G1-to-S phase cell cycle transition and are crucial for the onset, survival, and progression of breast cancer (BC). Small-molecule CDK4/CDK6 inhibitors (CDK4/6i) block phosphorylation of tumor suppressor Rb and thus restrain susceptible BC cells in G1 phase. Three CDK4/6i are approved for the first-line treatment of patients with advanced/metastatic hormone receptor-positive (HR+)/human epidermal growth factor receptor 2-negative (HER2-) BC in combination with endocrine therapy (ET). Though this has improved the clinical outcomes for survival of BC patients, there is no established standard next-line treatment to tackle drug resistance. Recent studies suggest that CDK4/6i can modulate other distinct effects in both BC and breast stromal compartments, which may provide new insights into aspects of their clinical activity. This review describes the biochemistry of the CDK4/6-Rb-E2F pathway in HR+ BC, then discusses how CDK4/6i can trigger other effects in BC/breast stromal compartments, and finally outlines the mechanisms of CDK4/6i resistance that have emerged in recent preclinical studies and clinical cohorts, emphasizing the impact of these findings on novel therapeutic opportunities in BC. • CDK4 and CDK6 are key molecules in the G1-to-S phase cell cycle transition and are crucial for the progression of breast cancer (BC). In this review we describe how small-molecule CDK4/6 inhibitors (CDK4/6i) block phosphorylation of Rb and thus restrain susceptible BC cells in G1 phase. • Three CDK4/6i have been FDA-approved for the first-line treatment of patients with advanced/metastatic HR+/HER2- BC in combination with endocrine therapy (ET): palbociclib, ribociclib, and abemaciclib. In this review we discuss in detail these three FDA-approved CDK4/6i. • CDK4/6i can modulate other distinct effects in both BC and breast stromal compartments, which may provide new insights into aspects of their clinical activity. In this review we describe the CDK4/6-Rb-E2F pathway in HR+ BC and then discuss how CDK4/6i can trigger other effects in BC/breast stromal compartments. [ABSTRACT FROM AUTHOR]