Venture capital (VC) firms spent over $25 billion funding clean energy technology (cleantech) start-ups from 2006 to 2011. Less than half of that capital was returned; as a result, funding has dried up in the cleantech sector. But as the International Energy Agency warns, without new energy technologies, the world cannot cost-effectively confront climate change. In this article, we present the most comprehensive account to date of the cleantech VC boom and bust. Our results aggregate hundreds of investments to calculate the risk and return profile of cleantech, and we compare the outcomes with those of medical and software technology investments. Cleantech posed high risks and yielded low returns to VCs. We conclude that among cleantech investments, "deep technology" investments in companies developing new hardware, materials, chemistries, or manufacturing processes consumed the most capital and yielded the lowest returns. We propose that broader support from policymakers, corporations, and investors is needed to underpin new innovation pathways for cleantech.