The purpose of this trial is to evaluate Herceptin treatment for 1 year and 2 years (versus observation/no Herceptin) in women with HER2-overexpressing primary breast cancer who have completed (neo-)adjuvant systemic chemotherapy, definitive surgery, and radiotherapy, if applicable. Efficacy and safety will be assessed for 10 years from randomization for each participant. All participants will continue to be followed for survival until 10 years after enrollment of the last participant.