Building on recent work for the 2009 UN Human Development Report, this chapter aims to explore one of the measures identified in a recent review by Bell et al. (Journal of the Royal Statistical Society A 165(3):435–464, 2002), as originally proposed by Courgeau (Population 28:511–537, 1973a). The particular appeal of Courgeau’s ‘k’ statistic is that it purports to provide a single summary index of migration intensity which transcends the differences in zonal systems that commonly confound cross-national comparisons. Courgeau’s k is applied to examine differences in mobility between 27 countries, using census data drawn primarily from the Integrated Public Use Microdata Series (IPUMS) database maintained by the University of Minnesota. Our goals of the chapter are threefold: first, to establish the strengths and limitations of Courgeau’s k as a summary measure of internal migration; second, to identify the extent of international differences in mobility; and third, to determine the general trajectory of internal migration over time.