This paper narrative argues that industrial economies have been locked into fossil fuel-based energy systems through a process of technological and institutional co-evolution driven by path-dependent increasing returns to scale. It is asserted that this condition, termed carbon lock-in, creates persistent market and policy failures that can inhibit the diffusion of carbon-saving technologies despite their apparent environmental and economic advantages. The notion of a Techno-Institutional Complex is introduced to capture the idea that lock-in occurs through combined interactions among technological systems and governing institutions. While carbon lock-in provides a conceptual basis for understanding macro-level barriers to the diffusion of carbon-saving technologies, it also generates questions for standard economic modeling approaches that abstract away technological and institutional evolution in their elaboration. The question of escaping carbon lock-in is left for a future paper.