This paper considers whether countries might mutually agree a policy of open borders, allowing free movement of workers across countries. For the countries to agree, the short run costs must outweighed by the long term benefits that result from better labor market flexibility and income smoothing. We show that such policies are less likely to be adopted when workers are less risk averse workers and when countries trade more. More surprisingly, we find that some congestion costs can help. This reverses the conventional wisdom that congestion costs tend to inhibit free migration policies.