This chapter outlines what design participation can offer for steering sector-wide systemic changes. Because of the difficulty of transforming sector-wide patterns of production and consumption to more sustainable footing in key societal sectors such as transport, energy, and land use, transitions governance is an increasingly central area of concern among policy, business, and design practitioners, as well as among academic researchers. Society-wide changes hold long-time frames and wide scope, spanning regulation, technology development, altered consumer practices, taxation, and new business creation (and so on). The chapter begins by introducing the basic facets of why such society-wide sociotechnical change is so difficult: path dependency, vested interests, technical and infrastructural lock-ins, and sector-wide and cross-sectoral interlinkages between different path dependencies forming “sociotechnical regimes.” It then moves to introduce the key transition governance approaches and how design participation can contribute to them. We elaborate these possibilities further through introducing the mid-range transition pathway toolkit (MTPT) and mid-range transition arenas. There, design participation fosters sector-wide changes by helping diverse actors in envisioning and building pathways toward a more sustainable future with hands-on tools and improved procedures. After briefly recounting the evolution of these tools to date, we shift gears and examine the process by which the MTPT was designed to exemplify those facets of intermediate designing that become foregrounded in codesigning for transitions. The chapter ends by considering the potential as well as the re-orientations that may be required from designers in catalyzing such transformative changes.