Inclusion Criteria:  Is male or female Has histologically or cytologically confirmed breast cancer diagnosis (HR+/HER2- breast cancer) Has received a diagnosis of locally advanced breast cancer not amenable to curative radiation or surgical cure or of metastatic breast cancer (i.e., advanced breast cancer [ABC]); diagnosis may be de novo (initial diagnosis at ABC stage) or recurrent from earlier-stage disease (initial diagnosis of earlier-stage disease with subsequent progression to advanced disease) Has initiated a required first-line therapy of interest: Palbociclib plus AI as first-line therapy for ABC between September 1, 2016, and July 31, 2020, or AI monotherapy as first-line therapy for ABC between January 1, 2010,, and July 31, 2020 Was aged 18 years or older at the time of diagnosis of ABC Is living or deceased at the time of record abstraction Has a complete medical record, covering treatment for ABC, including any transfer record from other facilities (if applicable) that is available to the abstractor for data abstraction  Exclusion Criteria:  The patient's first treatment after ABC diagnosis was chemotherapy (including induction chemotherapy) The patient has evidence of other active malignant neoplasms within 3 years (except nonmelanoma skin cancer or carcinoma in situ) prior to diagnosis of ABC; patients diagnosed with second primary cancer after ABC diagnosis will not be excluded The patient has evidence of prior treatment with any CDK4/6 inhibitor (i.e. palbociclib, abemaciclib, ribociclib) in the early-stage breast cancer setting or treatment with abemaciclib or ribociclib in the ABC setting The patient has participated in a clinical trial related to treatment of ABC (including after first-line therapy [i.e., palbociclib plus AI or AI monotherapy])