The world is urbanizing at an unprecedented rate, and its cities are transformed by technology and distributed computing. With every photograph, Twitter post, public transit ride, and credit card swipe, we leave digital traces in physical space. The enormous quantity of information, or Urban Big Data, that humanity generates each day is beginning to off er new possibilities for research, design, and systems optimization on the city scale, but the first step toward our urban future is finding new ways of understanding and visualizing Big Data—revealing invisible dimensions of the city.