Increasing urban density and increasing city size tend to reduce households’ average energy consumption. However, increasing population density also tends to reduce local air quality. This trade-off as well as urban policies to stimulate the transition towards low carbon cities are subject of study. This is done by collecting data in four very different European cities – Amsterdam, Istanbul, Gothenburg and Barcelona – and by developing a spatial-economic equilibrium model that can be used for policy simulations. BREATHE engages policy makers and companies from the four cities