By highlighting certain dynamics and certain economic stakes affecting transit areas as a result of Saharan and trans-Saharan migration, this article seeks to encourage these areas to be included systematically in the thinking about the migration-development relationship or the effect of migration policies. At the same time, working in the transit areas, and more particularly on transport, enables one to see migration not as a monolithic phenomenon, nor as a succession of fixed stages, but as a fluid process which spreads over time and space. Ultimately, understanding movement implies being immobile and observing those who pass by; being mobile and observing those who, through their immobility, appear to pass by; being mobile and observing those who, caught up in the same movement, appear immobile. These three approaches to migration, to movement in migration, are aimed at deciphering the tension between mobility and immobility, between travel, retention and temporary settlement, which is fundamental to all present-day migratory phenomena in North-West Africa. Since these moments in migration are not opposed to one another, but rather make up different facets of a single process, it is not a question of favouring research in the transit areas over that in the departure and destination areas, nor of favouring multiple-site or moving work over work which is fixed in a single location, but rather of stressing the complementary nature of these approaches with a view to gaining a general understanding of migratory phenomena.