The Biotic Ligand Model (BLM) attempts to predict metal toxicity to aquatic organisms on the basis of metal speciation and effects at the cell surface. Current versions of the BLM for silver and copper consider metal binding by inorganic ligands, dissolved organic matter (DOM) and also competition at the cell surface from calcium and protons (pH). Recent studies reported in the geochemical and ecotoxicological literature have indicated the importance of sulfide as a ligand, even in fully oxygenated aquatic systems. Speciation calculations for oxygenated waters do not currently include reduced sulfur as a ligand and as a consequence, no version of the BLM model has been published including reduced sulfur. This reflects the limitations on our knowledge regarding reduced sulfur in aquatic systems. In this paper we highlight the need to include reduced sulfur in the Biotic Ligand Model, with the interaction between silver and inorganic metal sulfides as a specific example. The geochemical importance of metal sulfides as ligands for silver and the effect of 'dissolved' metal sulfide and other ligands on metal toxicity and accumulation are described and reviewed. Recommendations are made for future work needed to incorporate sulfide ligands into the BLM's modeling framework.