The material impact of human life on earth continues to intensify, driven overwhelmingly by construction. This is of course not without consequence, including the substantial portion of carbon emissions from the production of building materials like concrete and steel. It is therefore necessary to understand material supply not as a naturalized law of supply and demand but as a key site for design and transformation. This contribution by Sarah Nichols will contextualize the factors that determine material choice in architecture by looking at material supply chains as large technological systems and, in complement, by looking at how conceptions about building materials are shaped—in short, at the radical transition needed in material supply as a technical, political, and cultural challenge.