The prospect of urban innovation excites the imagination. But dreaming up what a “smart city” will look like in some gleaming future is, by its nature, a utopian exercise. The messy truth is that cities are not the same, and even the most innovative approach can never achieve universal impact. What’s appealing for intellectuals in Copenhagen or Amsterdam is unlikely to help millions of workers in Jakarta or Lagos. To really make a difference, private entrepreneurs and civic entrepreneurs need to match projects to specific circumstances. An effective starting point is to break cities into four segments across two distinctions: legacy vs. new cities, and developed vs. emerging economies. The opportunities to innovate will differ greatly by segment.