Globally, internal migrants outnumber international migrants by 4 to 1 (Bell et al 2015) and recent years have seen significant progress in understanding internal migration in a comparative framework. The IMAGE project (Internal Migration Around the GlobE) developed a rigorous framework for cross-national comparisons of internal migration, involving (1) a suite of statistical indicators,(2) methods to generate estimates where comparable metrics are not collected directly, and (3) a global repository of internal migration data. The Aggregate Crude Migration Intensity (ACMI) captures the intensity of internal migration measuring all changes of address in a given interval. The Migration Effectiveness Index (MEI), which ranges from 0 to 100, quantifies the degree of balance between flows and counterflows, with low values indicating largely reciprocal exchanges between regions, while high values suggest strongly directional flows. Together, intensity and effectiveness drive the redistributive impact of migration on national populations.