This is a clinical practice guideline for female patients with metastatic breast cancer. The guideline examines whether biomarkers should be used to guide decisions on systemic therapy. Topics of interest include the clinical utility of sampling metastatic tumours to test for changes from the primary tumour with respect to endocrine receptor (ER/PR) or HER2 status, and whether additional clinical biomarkers beyond ER/PR/HER2 have shown clinical utility in initiating or directing clinical therapy, or guiding decisions on switching to a different regime or discontinuing treatment. Outcomes of interest include survival (overall, progression-free), quality of life, and cost effectiveness. The guideline examines whether changes to systemic therapy based on biomarker status improves health outcomes, quality of life and/or cost effectiveness. Specifically, the guideline reviews the testing of metastatic tumour for changes from primary tumour with respect to ER, PR, and HER2 status, whether there is evidence of additional tumour markers that have demonstrated clinical utility to initiate or direct selection of systemic therapy, or to switch or discontinue a specific drug regime.