Smartphone applications (or apps) are a relatively recent but already nearubiquitous media for engaging with the city, particularly in the global north. A search for ‘Dublin’ on Apple’s App Store and Google Play reveals a rich marketplace of apps for, amongst other things: catching up on local news, monitoring ight arrivals, navigating tourist attractions, negotiating bus timetables, nding bicycle or car hire services, paying for parking, discovering events and nding the right restaurant. Smartphone applications are a common, everyday way of exploring the city. For an urban geographer interested in the intersection of code and the city, smartphone applications oer a rich seam of research, but one that is not without its challenges. On the surface, applications oer a quite simple representation of information about a particular subject. But to perform this task they rely on a vast network of interlocking technologies with a complex spatial topology. This chapter oers a multilayered model, to open the black box, and provide a heuristic for critically examining mobile applications. The aim of this model is clear enough to be readily graspable, but not so reductive as to limit empirical engagement.