This is a clinical practice guideline for women with early-stage invasive breast cancer. The guideline provides recommendations on use of breast tumour biomarker assay results to inform decisions on adjuvant therapy. Several markers are reviewed including 21 gene recurrence score (Oncotype DX), 12 gene risk score (EndoPredict), 70 gene assay (MammaPrint), 50 gene subtype predictor (PAM50 Risk of Recurrence), Breast Cancer Index (BCI), 5 protein panel risk specification (Mammostrat), immunohistochemistry 4 (IHC-4), urokinase plasminogen activator and plasminogen activator inhibitor type 1 (uPA and PAI-1), circulating tumour cells (CTCs), tumour-infiltrating lymphocytes (TILs), and protein encoded by the MKI67 gene. Additionally, the guideline provides recommendations on the use of specific biomarkers to guide choice of specific drugs and regimes. Outcomes of interest include overall survival and disease-free or recurrence-free survival.