Environmental life-cycle assessment (LCA) of products has been the focus of growing attention in the last few years. The methodological framework has been developed rapidly, and a provisional “Code of Practice” has been drawn up by an international group of experts. One of the elements of LCA is impact assessment, which includes a characterization step in which the contributions of resource extraction and polluting emissions to impact categories such as resource depletion, global warming, and acidification are quantified and aggregated as far as possible. This can be achieved by multiplying extractions and emissions by a so-called equivalency factor and aggregating the results in one or more effect score(s) per impact category. In this report a proposal is developed for equivalency factors indicating the relative depletion of a resource per unit extracted. It is proposed to measure depletion by physical data on reserves and production and regeneration rates, and to distinguish between abiotic and biotic resources. Equations are developed to calculate equivalency factors for these two categories of resources, resulting in so-called abiotic depletion potentials (ADP) and biotic depletion potentials (BDP). The application of these ADPs and BDPs in LCA is illustrated.