Entrepreneurs and investors are making the most of the world’s excitement about clean technology, hammering out innovations in every energy-related field. Many of the projects show great promise for helping to meet the greenhouse-gas limits discussed at the recent UN climate summit in Copenhagen, but few people grasp this disturbing reality: Even if energy innovations have a lot of potential, they might not be deployable until it’s too late. History shows that most of the technology breakthroughs need decades to make it to the mass market.

A version of this article appeared in the January–February 2010 issue of Harvard Business Review.