This report describes the rationale, purpose and design of the POLARIS study. POLARIS is an ongoing noninterventional, prospective, multicenter study. Female and male patients in the USA and Canada diagnosed with hormone receptor-positive/HER2-negative metastatic breast cancer were enrolled in the study and treated with the cyclin-dependent kinase 4/6 inhibitor palbociclib when hormone receptor-positive/HER2-negative metastatic breast cancer was deemed to be indicated by their physician. The study will provide real-world data on palbociclib prescribing and treatment patterns in routine clinical practice, associated clinical outcomes, treatment sequencing in the advanced/metastatic setting, patient quality of life and geriatric-specific assessments. The tumor genomic landscape in relation to clinical outcomes will be explored. POLARIS will identify benefits and side effects of palbociclib across multiple lines of therapy and in discrete subsets of patients. Clinical Trial Registration: NCT03280303 ( ClinicalTrials.gov ).