The monoclonal antibody against HER2, trastuzumab (Herceptin), has become an important player in the treatment of patients with HER2-positive breast cancer. Both in the metastatic and adjuvant setting, the addition of trastuzumab to other systemic treatments has led to a striking increase in tumor response and survival. The downside with the use of this agent, however, is its inherent cardiotoxicity, which is particularly common when anthracyclines are used concurrently. This review will focus on all aspects of the cardiac side-effects of trastuzumab, ranging from epidemiology and pathophysiology to cardiac monitoring, and treatment and prevention.