Background: Additional billing is commonly and legally practiced in some countries for patients covered by health insurance. However, knowledge and understanding of the additional billings are limited. This study reviews evidence on additional billing practices including definition, scope of practice, regulations and their effects on insured patients. Methods: A systematic search of the full-text papers that provided the details of balance billing for health services, written in English, and published between 2000 and 2021 was carried out in Scopus, MEDLINE, EMBASE and Web of Science. Articles were screened independently by at least 2 reviewers for eligibility. Thematic analysis was applied. Results: In total, 94 studies were selected for the final analysis. Most of the included articles (83%) reported findings from the United States (US). Numerous terms of additional billings were used across countries such as balance billing, surprise billing, extra billing, supplements and out-of-pocket (OOP) spending. The range of services incurred these additional bills also varied across countries, insurance plans, and healthcare facilities; the frequently reported were emergency services, surgeries, and specialist consultation. There were a few positive though more studies reported negative effects of the substantial additional bills which undermined universal health coverage (UHC) goals by causing financial hardship and reducing access to care. A range of government measures had been applied to mitigate these adverse effects, but some difficulties still exist. Conclusion: Additional billings varied in terms of terminology, definitions, practices, profiles, regulations, and outcomes. There were a set of policy tools aimed to control substantial billing to insured patients despite some limitations and challenges. Governments should apply multiple policy measures to improve financial risk protection to the insured population.