Biotic ligand models (BLMs) have become a very valuable aid to assessing the risks posed by metals in the aquatic environment, because they enable us to understand the effect of water quality on metal toxicity. Graham and I, along with Steve Lofts at the Centre for Ecology and Hydrology, have been working with the Environment Agency and the International Manganese Institute to develop and validate BLMs for manganese for a fish, an invertebrate, and an algal species. These models will enable future water quality standards for manganese to be adjusted to take account of the effects of differences in local water chemistry on the potential toxicity of manganese to aquatic organisms. The different effects of water quality on manganese toxicity to primary producers (e.g. algae) and higher aquatic organisms (fish and invertebrates) mean that different species are likely to be the most sensitive to manganese toxicity under different types of water chemistry conditions. The models include two novel approaches towards addressing the toxicity of metals to aquatic organisms. The BLM for fish considers two sites at which manganese uptake occurs, and a single model is used to address both biomass and growth related effects on algae. The models have been validated in a variety of different water chemistry conditions. A paper detailing the development and validation of the models has recently been accepted for publication in Environmental Toxicology and Chemistry (abstract).