Trastuzumab is standard of care in the treatment of human epidermal growth factor receptor (HER)-2⁺ early and advanced breast cancer. Recently, it has been approved for the treatment of HER-2⁺ advanced gastric cancer. Trastuzumab is an IgG1 humanized monoclonal antibody administered by intravenous infusion on a weekly or three weekly schedule. In all registered indications, trastuzumab is almost always given in combination with chemotherapy. In hormonal receptor-positive breast cancer in postmenopausal women, trastuzumab can be combined with an aromatase inhibitor. Main toxicity is reduction in the left ventricular ejection fraction, which in a minority of patients can become symptomatic, but in many patients is at least partly reversible. Long-term safety needs to be further determined.