Migration is the movement of individuals in geographic space. When that movement has genetic consequences, migration determines gene flow. By introducing new genes where they were previously absent, migration increases genetic diversity within populations and reduces the differences among them. As a rule, groups that dwell close to each other tend to exchange more migrants than distant or isolated groups. But there are exceptions, especially during demographic expansions and when the populations are subdivided by geographic or cultural barriers. Rates and patterns of migration can be reconstructed from the analysis of genetic data.