This paper investigates the relation between mobility and fieldwork in geography. It is based on research on transit migration in the Central Sahara. Mobility is treated as the spatial dimension of fieldwork, as the opening of new grounds newly perceived which changes the kind of empiric data produced in migration studies. The paper aims to reflect on the mobility of the observer under two aspects : mobility in the field and mobility as fieldwork. Mobility in the field, i.e. movement between different but related field-sites, helps to contextualise each research locality and each phenomenon observed from different angles. At the same time, mobility is in itself a “field-site”, a privileged moment of observation and discussion. As a consequence, researchers themselves need to become mobile in order to study migration from the inside and to grasp the fleeting and unstable social constructions that mobility produces.