Informal payments are one of the components of cash payments which are often a form of corruption which is made outside the formal bill to health care providers. This study aims to review the documentation to identify the nature and reasons for this phenomenon. A Persian and English search without time and place limitations in electronic databases was conducted. Out of 15817 studies, 65 studies were consistent with quality assessment. Different characteristics for these payments are expressed, but there is no universal definition for them. Almost all of the studies referred to the illegal nature of this phenomenon. Previously, it was considered a social phenomenon, but today, lack of resources, poor management, governance, and political weaknesses, are also accompanied by cultural and social reasons. The most popular labels are informal payments, gratitude payments, envelope payments, unofficial payments, bribes payments, under-the-table payments, and red package payments. Incentives cover a wide range from an appreciation to obligation. The purpose of these payments is for higher quality or improved access, and gift or appreciation. Individual characteristics of the recipients, the organizational features of the provider, the nature of services can affect the probability of occurrence. These payments have a negative impact on the health system and consequently patients and even providers. Strategies such as using a legal approach, increasing knowledge of the people, changing the culture, improving quality of services and careful monitoring of the delivery process and improving management and transparency in health system can be effective in reduction of these payments.