This document results from an OECD project on Aquatic Effects Assessment. In the scheme developed in the document, information on the toxicity of a chemical is used to derive a Maximum Tolerable Concentration (MTG) in water, i.e. the maximum concentration of a chemical at which no unacceptable adverse effects on the ecosystem are expected. Various ways of deriving an MTG are described. They depend on the type and extent of the data available. If a limited toxicity data set is available (e.g. data on fewer than five species), various assessment factors are recommended for adjusting the effects concentration (e.g. L(E)C50, No Observed Effect Concentration - NOEC, etc.) and for deriving an MTG. Where laboratory data are missing, Quantitative Structure Activity Relationships (QSARs) may be used to predict toxicity. QSARs quantitatively relate aquatic toxicity to structural characteristics and/or physical-chemical properties (e.g. octanol-water partition coefficient - K0w) of a substance. This document describes two approaches which may be used, one based on the mode of action of the chemical, the other based on chemical classes.