The aim of this review article is to examine the available evidence regarding diagnosis and treatment of HER-2 positive breast cancer. This group of breast tumours (up to 30% of the total number of breast cancers) is known for having a more aggressive behaviour. The current recommendations for HER-2 positive tumour diagnosis are discussed since accurate identification of HER-2 amplification or overexpression is key for allowing a correct risk assessment and treatment. HER-2 positive tumours can be treated with trastuzumab (Herceptin, Hoffmann-La Roche, Basel, Switzerland), a monoclonal antibody targeted against the HER-2 receptor. The role of this drug in the metastatic, adjuvant and neoadjuvant setting is reviewed. The results of the recently reported adjuvant trials are commented, as the positive results of these trials changed the standard of care for patients with this particular type of breast cancer.