Inclusion Criteria:  ≥18 years or older with diagnosis of adenocarcinoma of the breast with evidence of metastatic /locally advanced disease not amenable to treatment with curative intent. Documented HR+ (ER+ and/or PR+) tumor based on local standards Documented HER2- tumor based on local standards Will initiate treatment with palbociclib plus letrozole/aromatase inhibitor or palbociclib plus fulvestrant in line with the licensed indication(s), as first or second line therapy for metastatic/locally advanced BC at enrollment Patients who in the opinion of the investigator are willing and able to comply with regular clinic visits Evidence of a personally signed and dated informed consent document indicating that the patient (or a legally acceptable representative) has been informed of all pertinent aspects of the study  Exclusion Criteria:  Patients participating in any interventional clinical trial Patients on active treatment for malignancies other than metastatic/locally advanced BC at the time of enrollment Patients who are unable to understand the nature of the study and are unwilling to sign an informed consent