In this fascinating new field, this paper offers some reflections on portability of social benefits, and tries to identify promising research topics. These reflections are organized in three sections. The first analyzes the channels that underpin the positive value of portability of social benefits in models of international trade, and with regards to fairness, analyzes different welfare criteria for assessing migration, with special attention to the human rights of migrants. The second section focuses on migration by active workers. The final section focuses on retiree migration, which is inherently simpler because it does not have a direct impact on the labour market.