The main purpose of this study is to compare progression-free survival for women with hormone receptor positive (HR+), human epidermal growth factor receptor (HER2) negative advanced breast cancer receiving either abemaciclib + fulvestrant or fulvestrant alone. Participants will be randomized to abemaciclib or placebo in a 2:1 ratio. The study will last about 9 months for each participant. For the endocrine naïve cohort, all participants will received abemaciclib + fulvestrant.