As I write this essay, I feel myself being drawn inexorably toward one of the world's great destinations: New York City. Okay, I'm actually riding a commuter train. But this daily journey always feels compelling to me. I'm headed toward a place of great energy, where I work and find collaborative opportunities, meet up with friends, enjoy cultural activities and often find myself spontaneously marveling at the surrounding man-made wonders.

Many other people clearly are equally captivated by the opportunities they find in cities. That is why more than half of humanity lives in these centers of enterprise and innovation, with that number rising quickly. An estimated five billion will be dwelling in cities by 2030—and half of them will be moving into homes and workplaces that do not yet exist. How we create and reshape our urban landscapes and systems will have a powerful effect on the future of our world: “As cities go, so goes the planet,” writes architect William McDonough in “How Cities Could Save Us.” This article is part of a special report that takes a look at sustainable cities. One key, McDonough says, is to think in terms of living systems: circular systems, rather than linear. As he puts it, “Cities are designed, but they are also organisms.” Following the theme, energy researcher Michael E. Webber explores “Tapping the Trash.” He describes how urban centers can help us reduce and reuse waste heat, water and materials, creating revenue and lowering costs. The third part of our report looks at how we get around: Carlo Ratti and Assaf Biderman, both at the Massachusetts Institute of Technology, consider a turning point for transportation enabled by webs of sensor-laden vehicles and smart intersections in their feature, “From Parking Lot to Paradise.”

We each carry within ourselves a different kind of city: an inner hub of energetic neural activity that creates our mind. In the cover story, “Memory's Intricate Web,” neuroscientist Alcino J. Silva describes nothing less than a revolution in memory research wrought by new technologies that can image the activity of individual neurons—even switching the cells on and off as directed. Learning more about how specific cells store a given memory is giving us insights into how we mentally construct the world around us, which for me feels resonant in an issue in which we are discussing how we are shaping the planet. As ever, we welcome your comments.