This report describes and discusses approaches for the environmental risk assessment (aquatic compartment) of mixtures under ⁠REACH⁠. Different types of mixtures are defined. The focus lies on technical mixtures and discharge mixtures. Cumulative and aggregated exposures are considered. Interfaces to other regulations (e.g. Water Framework Directive) and consideration of substances not regulated under REACH (e.g. biocides) are briefly discussed.