Patient complaints may help identify complications during hospitalization. For example, the official investigation of the Mid-Staffordshire NHS Foundation Trust in the United Kingdom found that unsolicited written patient complaints had identified many areas of poor care that likely contributed to preventable patient deaths. This systematic review sought to create a common taxonomy for patient complaints in order to standardize future research and analyses. The taxonomy includes three main domains—clinical, management, and relationships—which are parsed into seven categories and further sub-categories. This taxonomy may enable comparisons between health care institutions and more sophisticated aggregate analyses of patient complaints, which in turn could contribute to better understanding about the value of patient complaints and the effectiveness of different interventions aimed at addressing patient concerns.