Purpose: Comorbidity in breast cancer patients impacts treatment choice, toxicity, and outcomes. While comorbidity measurement tools are frequently used by researchers, little is known about their use in clinical practice. The aim of this review was to examine the use of comorbidity measurement tools in clinical practice and their role in treatment decision-making in breast cancer. Methods: Six electronic databases were searched from inception to 21 March 2019. Quantitative or mixed methods studies addressing primary treatment of breast cancer and identifying a comorbidity measurement tool used in clinical practice treatment decision-making were included. Data was extracted on tool utilized, impact on treatment decisions or outcomes, pattern of use, and psychometric properties. Results: A systematic search of literature yielded 752 studies. Of the four studies that met inclusion criteria, each utilized a comprehensive geriatric assessment tool, though only in a subset of patients. No studies found direct comorbidity measurement tools utilized independently of geriatric assessment. Assessment results had variable impact on treatment decisions. Impacts on patient mortality and treatment toxicity, cost-effectiveness, and psychometric characteristics of the tools were not identified. Conclusions: There is little evidence on use of specific comorbidity tools in clinical decision-making in breast cancer outside of comorbidity assessment as part of geriatric assessment tools. There was limited impact on decision-making or patient outcomes when these were utilized. Further research is needed to identify barriers to comorbidity assessment in clinical practice and identify comorbidity tools that have the potential to improve patient outcomes.