Venture capital (VC) firms spent over $25 billion funding clean energy technology (cleantech) start-ups from 2006 to 2011 and lost over half their money; as a result, funding has dried up in the cleantech sector. In this article, we present the most comprehensive account to date of the cleantech VC boom and bust, aggregating hundreds of investments to calculate the risk/return profile of cleantech, compared with those of medical and software technology investments. The results are stark— cleantech offered VCs a dismal risk/return profile, dragged down by companies developing new materials, chemistries, or processes that never achieved manufacturing scale. We conclude that the VC model is broken for the cleantech sector, which suffers especially from a dearth of large corporations willing to invest in innovation. Fortunately, new public and private capital may be on the way after announcements made at the 2015 Paris Climate Change Summit. If a new and more diverse set of actors avoids the mistakes of the cleantech VC boom and bust, then they may be able to support a new generation of cleantech companies.