This multi-center, prospective observational study will evaluate the treatment patterns, and the safety in patients with HER2-positive (HER2+) metastatic breast cancer (MBC). Eligible patients will have an initial metastatic breast cancer diagnosis that has not been previously treated with systemic therapy; patients may be enrolled up to 6 months after the diagnosis. Data will be collected for up to 8 years.

Inclusion Criteria:

• Adult patients, >/=18 years of age
• Initial diagnosis of HER2+ MBC, as diagnosed by the treating physician, no more than 6 months prior to study enrollment
• Availability of cancer-specific historical data points in the patient's medical records

Exclusion Criteria:

• Any inability to provide informed consent

Patients with HER2 positive (HER2+) metastatic breast cancer