We ask what level of migration would maximize world welfare. Welfare is assumed to be a weighted average of the utilities of the world’s various citizens, but the weights are also country specific. Using a calibrated one-sector model, we find that unless the weights are heavily biased toward the natives of rich countries, the extent of migration that would be optimal far exceeds the levels observed today. The claim remains true in a two-sector extension of the model. All versions of the model assume that migration is the only redistributive tool.