HER2 is overexpressed or gene amplified in 20%-25% of breast cancers. The anti-HER2 monoclonal antibody trastuzumab targets HER2-positive tumors, inhibiting proliferation and inducing cell death via extracellular and intracellular mechanisms. The clinical benefits observed with trastuzumab in the metastatic setting provided the rationale for assessing trastuzumab in the treatment of early breast cancer. Four large phase III adjuvant trials (NSABP B-31, NCCTG N9831, HERA, and BCIRG 006) investigated the efficacy and safety of 1 or 2 years of trastuzumab given in combination with or after standard adjuvant chemotherapy. The addition of 1 year of trastuzumab to adjuvant chemotherapy significantly improved disease-free survival (DFS) by 33%-52% and overall survival by 34%-41% in the 4 trials. The DFS benefits were observed regardless of age, nodal status, hormonal status, or tumor size in all trials. The cumulative incidence of congestive heart failure or cardiac death ranged from 0-0.9% in the control arms and 0-3.8% in the trastuzumab-containing arms. These were below the safety cutoff points set by the individual studies' independent data monitoring committees, indicating acceptable cardiac safety. Risk factors associated with cardiac dysfunction included baseline left ventricular ejection fraction level, hypertension, and older age. The addition of trastuzumab to adjuvant chemotherapy provides significant survival benefits with a positive benefit/risk ratio. Ongoing and planned trials correlated with basic science will enhance our understanding of HER2-positive disease, leading to treatment optimization and further improvements in patient outcomes.