The Hirsch-index is a well-known index for measuring and comparing the output of scientific researchers. The main contribution of this article is an axiomatic characterization of the Hirsch-index in terms of three natural axioms. Furthermore, two other scientific impact indices (called the w-index and the maximum-index) are defined and characterized in terms of similar axioms. (C) 2008 Elsevier B.V. All rights reserved.