This paper analyzes to what extent different indices are able to measure the concentration in air transport networks. First, we illustrate the theoretical characteristics of the concentration ratio, Herfindahl-Hirschman index, network concentration index and Theilindex by applying these to socalled toy networks with known parameters. Second, the practical applicability of these indices is assessed by calculating them for three domestic airline networks (France, Germany and the United States). The dimensions of the concentration in each of these three networks as a proxy for the city systems are intuitively clear so that the results provide us with an appropriate touchstone for assessing the advantages and drawbacks of each of the indices. Based on the analyses of both the ideal-typical and the actually existing networks, the main implications for future research on concentration indices are discussed
