This paper addresses issues of material flows in a selection of modern steel plants, especially the generation of wastes, from a social sciences perspective. I analyse key factors structuring waste management decisions through the case of “problem” wastes arising at a steel company’s plants. I discuss how some materials come to be construed as more problematical than others from a material and technological point of view, but also by taking into account organisational and legislative issues, in order to show that the construction of the “waste” category needs to be envisioned as resulting from a matrix of socio-material causes.