This real-life Health Economics and Outcome Research (HEOR) study will enable to assess the impact of current therapies on quality of life (QoL), productivity loss and health care resource utilization in metastatic breast cancer (mBC). This prospective study will estimate patient-reported outcomes (PROs) and resource utilization data for mBC patients stratified according to treatment type, treatment line and disease status (progression vs. progression free) in a real-life setting.