This paper reviews different approaches described in the literature for estimating the aquatic toxicity of chemical substances. It is based on an extended review performed by the European Chemicals Bureau of the European Commissions Joint Research Centre in support of the development of technical guidance for the implementation of the REACH legislation, and is one of a series of minireviews in this journal. The paper is organised by approach for (Q)SAR development and includes a review of: (i) (Q)SARs for acute aquatic toxicity by chemical class, (ii) (Q)SARs for acute aquatic toxicity by mode of action, (iii) review of statistically derived (Q)SARs, (iv) structural alerts for excess aquatic toxicity and (v) expert systems that combine structural rules and multiple (Q)SAR models to predict aquatic toxicological endpoints. Directions and recommendations for further research are also provided.