Background and Aim : Nowadays systems are not only responsible for improving the of people, but also to protect them against illness and illness costs. Hence, one of the challenges faced by governments is the reduction of out of pocket to care. Unofficial or informal are one kind of out of pocket that include to individuals or service providers, in cash or non-cash beyond formal processes. According to statistics and evidences, between 10 to 45 percent of the total out of pocket across low-income countries are informal. Addressing this phenomenon requires information about such and their roots. Owing to high prevalence of informal through many systems and nations, this systematic review aimed at identifying the nature, types of informal and their causes. Materials and Methods : In this systematic review, the keywords of unofficial payments or illegal payments or under-the table payment or Bribe and health in the Iranian databases of SID, IranMedex, Magiran and international databases of PubMed, Scopus, Science Direct, and Health Management Information Consortium (HMIC) were searched between years of 2000 to 2016. Also Google Scholar was used to find relevant resources and making sure about the search strategy coverage. Exclusion criteria included studies outside the system studies published in languages other than Persian or English. Two researcher independently selected papers based on inclusion and exclusion criteria and disagreements were resolved through consultation between them. Also bibliographic search was done to find possible relevant studies. The result was 58 articles. After reviewing the titles and abstracts of the articles, 13 were excluded on relevance, 9 due to duplication and 3 because of language. Finally, 33 articles were included for the final analysis. Microsoft Excel was used for data extraction. The data were analyzed based on the emerging themes narratively. Ethical Considerations : This study was approved by the Ethics Committee of the Vice-Chancellor of Research, Iran University of Medical Sciences (IUMS / 93-04-16-25284). Integrity and trustworthiness in selecting of articles, reporting of findings and referencing were considered. Findings: Thirty-three full papers were selected for study and analysis. Seven of the studies were related to Iran and 26 of them were related to other countries, especially the Eastern European nations. No study was found from North America nor the Western Europe. Most of studies were conducted in higher-income countries. Studies described different characteristics for informal payments, so there was no uniform definition for this phenomenon. These are either made in cash or non-cash. Most cash are made before and through receiving services and non-cash when service is received. The causes of informal were categorized into three groups. First, payers, namely, patients and their families, wish to receive higher quality services, better access to services or in order to saving time, such as the waiting time for surgery and appointment of a doctor visit. Second, from the caregivers’ perspective, especially physicians, was low wages and tariffs, and irregular deductions from insurance companies. Finally, the system's weaknesses as the third reason include insufficiency of insurance coverage, inadequate monitoring of tariffs and the lack control over charges, lack of human resources in some cases, and the asymmetry of information between provider and patient. The job and income level, level of education, age, gender and place of residence (rural or urban) of the patient and his/her family are factors influencing the amount and volume of informal payments. Among the doctors, the reputation and type of expertise are the most important determinants, so that surgeons and gynecologists had more informal inquiries in comparison to internists and psychiatrists. Also, doctors asked more money in urgent and critical cases. Famous doctors as well as doctors who worked in big cities received more informal payments. Also, the amount of were more in hospitals, inpatient wards and in sophisticated treatments and those used higher technology and modern facilities. Conclusion : Informal are not defined universally, and attitudes toward them are different among different societies, because of different norms, resulted from different cultural and social values of societies. Informal payments, with any intention and for whatever reason, have generally certain negative consequences on the systems and can affect patients and even service providers. Catastrophic costs, inequality of access to services and discriminated access to high quality care, and inequality in outcomes are the main consequences of informal payments. The major impact, however, is to limitation of patients’ access to care due to their higher costs. This result in inequalities in outcomes. In addition, social justice will be vanished as one of the main responsibilities and missions of states, which will lead to the loss of public confidence in the system. Revisions in the systems along with the redefinition of charges, reinforcement of insurance systems, informing patients about their rights and improving control over the reduction of informal are recommended. Please cite this article as: Jahangiri R, Aryankhesal A. Factors Influencing on Informal Payments in Healthcare Systems: A Systematic Review. Med Ethics J 2017; 11(40): 73-92.