Techno-optimism can be broadly defined as the belief that science and technology will be able to solve the major social and environmental problems of our times, without fundamentally rethinking the structure or goals of our growth-based economies or the nature of Western-style, affluent lifestyles. In other words, techno-optimism is the belief that the problems caused by economic growth can be solved by more growth (as measured by GDP), provided we learn how to produce and consume more efficiently through the application of science and technology. After providing a theoretical framework, this chapter presents an evidence-based critique of such techno-optimism, arguing that the vision of progress it promotes is unrealisable due to the limits of technology and the inherent structure of growth economics. The poor historical record on “decoupling” GDP from environmental impacts is examined, and this analysis is used to explain why efficiency improvements have not produced sustainable economies despite extraordinary technological advance in recent decades. It turns out efficiency improvements have not often been able to keep up with continued economic and population growth, meaning that overall global environmental impacts continue to grow, despite efficiency improvements.