The Special Issue collects six papers that use mobile phone data and spatial analysis techniques to study new urban critical features and social phenomena that arose with the Covid-19 pandemic. The applications of mobile phone data in the three study contexts investigated in this special issue highlight the potentialities of mobile phone data, as well as their limits. Compared to traditional methods of urban survey (such as census data or interviews), mobile phone data provide real-time maps of daily practices, thanks to valuable information on transient populations and their distribution over spaces.