Human epidermal growth factor receptor 2 is a member of human epidermal growth factor receptors, overexpressed or amplified in nearly 20% of breast cancer cases. HER2 overexpression is associated with a more aggressive disease phenotype, with progression-free and overall survival rates worse than patients who were HER2-negative. Trastuzumab was the first HER2-targeted agent introduced in breast cancer and its introduction led to a change of the natural history of HER2-positive metastatic breast cancer, which acquired more favourable outcomes due to the efficacy of trastuzumab. In early HER2-positive breast cancer trastuzumab, administered for 1 year according to the international guidelines, significantly improved disease-free survival and distant disease-free survival as compared to non-trastuzumab containing regimens. In metastatic breast cancer, despite some activity as a monotherapy, early addition of trastuzumab to cytotoxic chemotherapy markedly improved both the response rates and the overall survival. First generation studies identified the trastuzumab plus taxane combination as the gold standard for the first line treatment of metastatic breast cancer patients. Second generation studies showed, in combination with chemotherapy, the superiority of dual-blockade of HER2 over trastuzumab alone. Trastuzumab biosimilars showed the same overall response as trastuzumab in the phase 3. Therefore, these trials do support the use of trastuzumab biosimilars for the treatment of brest cancer patients.