Reurbanization refers to the new demographic growth of cities that previously lost population. How can we explain such a trend reversal? This paper discusses theoretically both phases of urban decline and reurbanization. It examines the trajectory of Swiss cities that have moved from a period of decline (1970-2000) to a new growth (since 2000). It analyses the components behind the population evolution, discusses the socio-cultural, economic and political trends triggering or enabling reurbanization, and identifies three main results. First, reurbanization is due to several population groups: the growing international mobility of the labour force, the increasing number of non-family households (in the context of the second demographic transition) and the growing attractiveness of cities for young adults (extension of youth as a life stage). Second, reurbanization is not only housing-led (construction of dwellings due to planning strategies and real estate activities). It is also population-led: A generation replacement induces a rejuvanation of the age structure and an increase in the density of occupancy of dwellings. Third, reurbanization can be broadly interpreted as a return of cities in terms of residential aspirations, political agenda and real estate activities. As a conclusion, I outline a research agenda on reurbanization.