There is increasing interest in the issue of informal payments for health care in low- and middle-income countries. Emerging evidence suggests that the phenomenon is both diverse, including many variants from cash payments to in-kind contributions and from gift giving to informal charging, and widespread, reported from countries in at least three continents. However, cross-national research is hampered by the lack of consensus among researchers on the definition of informal payments, and the definitions that have been proposed are unable to incorporate all forms of the phenomenon that have been described so far. This article aims to overcome this limitation by proposing a new definition based on the concept of entitlement for services. First, the various forms of informal payment observed in practice are reviewed briefly. Then, some of the proposed definitions are discussed, pointing out that none of the distinctive characteristics implied by these definitions, including illegality, informality, and corruption, is adequate to capture all varieties of the phenomenon. Next, an alternative definition is formulated, which identifies the distinctive feature common to all forms of informal payments as something that is contributed in addition to the terms of entitlement. Then, the boundaries implied by this definition are explored and, finally, the implications for research and policy making are discussed with reference to the lessons developed countries can learn from the experiences of transitional countries.