This paper develops the concept of‘action space’ as the range of possible destinations towhich a migrant can realistically move at a given point intime and, intimately linked to this, the set of possiblelivelihoods at destination. It shows how this space expandsand contracts over time through ‘cumulative causation.’ Sucha dynamic framework allows for appreciating the role ofsecondary towns in rural-urban migration and povertyreduction. Secondary towns occupy a unique middle groundbetween semi-subsistence agriculture and the capitalisticcity, between what is close by and familiar and what is muchfurther away and unknown. By opening the horizons of the(poorer) rural population and facilitating navigation of thenonfarm economy, secondary towns allow a broader base of thepoor population to become physically, economically, andsocially mobile. Secondary towns therefore have greatpotential as vehicles for inclusive growth and povertyreduction in urbanizing developing countries. These are theinsights emerging from the in-depth life history accounts of75 purposively selected rural-urban migrants from ruralKagera, in Tanzania.