At present there is no unified treatment, drawing together models to allow a consistent and reliable set of migration flows, across countries. This text seeks to do exactly that, potentially improving policies, planning and understanding about migration processes worldwide, via the presentation of migration estimation and modeling techniques. These modeling techniques are explored from both frequentist and Bayesian perspectives. The vital concepts such as missing data and collection methods (and their possible harmonization) are discussed in depth, and there are whole chapters dedicated to both modeling asylum flows and forecasts about the future of international migration.