Urban economics study how individual and company decisions on location are made and also study resulting spatial disparities, e.g. in terms of nominal and real income. City size plays a crucial role. However, the literature, mainly focused on cross-sectional differences, has as yet remained relatively silent on the sources of population growth in cities over time. Cities may grow as a result of high fertility coupled with low mobility of individuals, or as a result of strong migration flows, attracting workers born in other cities. This research project proposes to model the growth of cities by incorporating both fertility and migration decisions made by individuals.