This article provides a sectoral innovation system perspective of the development of energy efficient and clean process technologies in the Swedish pulp and paper industry. Specifically, the analysis elaborates the importance of knowledge development, actor networks, and institutions (including policy) for progressing and diffusing novel technologies related to energy use. The empirical analysis also sheds light on how significant changes in the sectoral innovation system have influenced the relevant research, development and demonstration activities in the Swedish pulp and paper industry over the period 1970–2010. The results are based on various sources—e.g., industry magazines, reports from industrial consultants and associations, minutes from meetings—and illustrate the importance of well-functioning innovation systems for successful technological development and diffusion processes. They display, in particular, the importance of joint, industry-wide R&D activities, trust-based state—industry relationships, government R&D expenditures, and intense information sharing. One important implication is that the role of policy stretches beyond the funding of basic R&D. Policy also involves measures that strengthen existing actor networks, build competence, and secure the existence of research institutes that provide a bridge between basic knowledge generation (at the universities) on the one hand, and industrial application on the other.