Clinical Ethics Committees (CECs), as distinct from Research Ethics Committees, were originally established with the aim of supporting healthcare professionals in managing controversial clinical ethical issues. However, it is still unclear whether they manage to accomplish this task and what is their impact on clinical practice. This systematic review aims to collect available assessments of CECs’ performance as reported in literature, in order to evaluate CECs’ effectiveness. We retrieved all literature published up to November 2019 in six databases (PubMed, Ovid MEDLINE, Scopus, Philosopher’s Index, Embase and Web of Science), following PRISMA guidelines. We included only articles specifically addressing CECs and providing any form of CECs performance assessment. Twenty-nine articles were included. Ethics consultation was the most evaluated of CECs’ functions. We did not find standardized tools for measuring CECs’ efficacy, but 33% of studies considered “user satisfaction” as an indicator, with 94% of them reporting an average positive perception of CECs’ impact. Changes in patient treatment and a decrease of moral distress in health personnel were reported as additional outcomes of ethics consultation. The highly diverse ways by which CECs carry out their activities make CECs’ evaluation difficult. The adoption of shared criteria would be desirable to provide a reliable answer to the question about their effectiveness. Nonetheless, in general both users and providers consider CECs as helpful, relevant to their work, able to improve the quality of care. Their main function is ethics consultation, while less attention seems to be devoted to bioethics education and policy formation.