Informal payments are known to be widespread in the post-communist health care systems of Central and Eastern Europe. However, their role and nature remains contentious with the debate characterized by much polemic. This paper aims to make sense of this debate by reviewing and summarizing the main arguments of the theoretical debate in Hungary. The review examines the possible causes of informal payment, the motivation of the actors involved and the impact of informal payment on system performance, focusing on efficiency and equity. The lines of arguments are summarized in two contrasting hypotheses, which envisage informal payment as either a donation or a fee-for-service. Evidence pertaining to the scale of informal payments and the motivation of patients are reviewed, but found to be inconclusive to verify the hypotheses. Although focused on Hungary, accounts from other countries facing informal payments show similar threads of discussion and dilemmas. These theories should be tested further using evidence from existing studies and new empirical research, since the validity of the gratitude payment concept is a central dilemma of effective policy making in the area. To orient future research, a possible agenda is outlined, which links evidence to be obtained to the defining features of gratitude payments.