The Project deals with the possibilities of performing mixture toxicity assessment within the environmental risk assessment of the authorization of biocide and plant protection products. To this end a review on the state of scientific knowledge about the predictivity of combined effects is collated. Central in this context is the reference model of Concentration Addition which allows extrapolating combined effects for mixtures based on knowledge about the effects of the components. Building on this, options for risk regulation are developed. Their applicability is considered in the context of those data that are currently available within the authorization process for biocide and plant protection products. Deficits with respect to a – scientifically sensible – homogeneous data base can often be overcome with pragmatic decisions if additional requirements for the authorization process are not an option. Tiered schemes to specifically account for combined effects during environmental risk assessment of biocide and plant protection product authorization are suggested, accompanied with a software tool for its implementation.