Abemaciclib is a selective cyclin-dependent kinase(CDK)4 & 6 inhibitor, which induces G1 cell cycle arrest and tumour growth inhibition. Abemaciclib has been developed for use in hormone receptor positive(HR+)breast cancer, dosed daily in combination with endocrine therapy. In a phase Ⅲ clinical trial, MONARCH 2, for women with HR+ and human epidermal growth factor receptor 2 negative(HER2-)advanced breast cancer who progressed after endocrine therapy, abemaciclib in combination with fulvestrant significantly improved not only progression-free survival and objective response rate but also overall survival, and demonstrated a tolerable safety profile. Another phase Ⅲ clinical trial, MONARCH 3, for women with HR+ and HER2- advanced breast cancer, abemaciclib in combination with nonsteroidal aromatase inhibitor as an initial therapy also significantly improved progression-free survival and objective response rate. This review presents the rationale for the use of CDK4 & 6 inhibitors in the treatment of breast cancer, background on the development of abemaciclib, clinical data focusing on phase Ⅲ studies of abemaciclib, and information on ongoing clinical studies of abemaciclib.