On the one hand, increasing urban density and increasing city size tends to have the positive effect of reducing the average household’s energy consumption. On the other hand, increasing population density also has the negative effect of reducing air quality. This trade-off is a tricky one. This dilemma combined with urban policies designed to stimulate the transition towards low carbon cities are a fascinating area of study. BREATHE is a project that explores this by analysing the interactions between urban form, economic welfare, energy use, and emissions by households and firms. This is done by gathering data from the cities of Amsterdam, Istanbul, Gothenburg, and Barcelona. This data is then used to develop a spatial-economic equilibrium model that can be used for policy simulations. BREATHE does this by actively engaging with policy makers and companies from these four cities to produce the most representative and accurate result possible.