Inclusion Criteria:  Adult female or male patients ≥18 years old or 'adults' according to age of majority as defined by the local regulations Patient or next of kin/legal representative willing and able to provide written informed consent according to the local regulations unless a waiver is granted by the local IRB/IEC/EC Patients' medical records showing diagnosis of HER2-positive unresectable or mBC (can be either de novo advanced disease, progression or recurrence of previous early-stage HER2-positive BC) since the available date of T-DM1 (Kadcyla) through reimbursement or patient access programme as a valid local treatment option or 01 January 2017, whichever is earlier, and with availability of at least 12 months of follow-up data (from the date of diagnosis of unresectable or mBC) in the medical records at the participating site, unless patient died within the first 12 months of diagnosis Patients completing at least 2 LOTs for HER2-positive unresectable or mBC  Exclusion Criteria:  Patients with HER2-negative unresectable or mBC at index diagnosis Patients with a change in HER2 status from positive to negative at progression from early-stage to advanced-stage disease (ie, shown on a repeat biopsy at diagnosis of advanced-stage disease) will be excluded (patients who change from HER2-positive to negative on repeat biopsy during treatment for advanced-stage disease may be included) Patients with a concomitant cancer at the time of diagnosis of HER2-positive unresectable or mBC except for the non-metastatic non-melanoma skin cancers, or in situ, or benign neoplasms; a cancer is considered concomitant if it occurs within 5 years of HER2-positive breast cancer diagnosis Patients who at time of data collection for this study are participating or have participated in an interventional study that remains blinded