Sustainability has become a byword. Whilst it is understood that we are now mainly an urban species, that the overwhelming majority of economic activity and associated resource use takes place in cities, some surprisingly basic questions remain unanswered: How do we define city sustainability? How do we then measure it to determine how sustainable a city is? Which are the most effective strategies and policy measures to bring about positive change? How do we model them to evaluate their effectiveness? How do we then implement them? These are some of the questions currently being tackled by the Laboratory of Urban Complexity and Sustainability (LUCAS) at Nottingham. Positioned within this landscape, I will focus in this talk on the challenges we face in developing a platform for modelling the principle urban resource flows, in a sufficiently spatially sensitive manner to support the testing of scenarios to minimise these flows (or some measure(s) of them) in the future and how cities might transition to these target future states.