Background Biocides not only kill pathogens that are targeted, but can also kill non-pathogens. This can have undesirable side-effects for human health and the environment. Such risks have yet to be sufficiently investigated for most biocides on the EU market because in different environmental compartments (e.g. soil, groundwater and air) active substances undergo degradation to metabolites that may be more toxic than the original compound. In this context, novel tools are necessary for the identification and substitution of biocides of ecotoxicological concern. Computational toxicology is a subdiscipline of toxicology that aims to use mathematics, statistics, chemistry and computer modelling to predict the toxic effects of chemicals on human health and the environment. Predictive toxicology is of increasingly interest due to new legal requirements imposed by the Biocides Product Regulation, Regulation (EU) 258/2012 (BPR). Under this legislation, a large amount of animal (in vivo) testing is needed to demonstrate the safety of new chemical compounds, requiring much time and cost as well as raising ethical questions. The use of alternative non-animal methods (e.g. computer models), as foreseen under the BPR, would reduce the need for animal testing. The availability of such methods would allow the screening of many chemicals in a simplified way, and would particularly help SMEs identify metabolites of ecotoxicological concern to facilitate the registering of biocides under the BPR. Objectives The LIFE-COMBASE project will demonstrate a new computational tool for assessing and reducing the impact of biocides of ecotoxicological concern, and will promote their substitution with safer substances. This flexible, open-source, online decision-support tool, for assessing the ecotoxicity of both biocidal products and their metabolites, will be based on a series of predictive in silico (computational) models; building on previous initiatives (namely, the projects ANTARES, CALEIDOS and PROSIL LIFE). The tool will enable simulations to be performed on the ecotoxicity potential of new candidate chemicals before their synthesis in a safe, residue-free and environmentally friendly way. Its operability and effectiveness will be demonstrated at four trophic levels: bacteria, algae, common water fleas (Daphnia) and fish. Project results will be communicated to the Biocidal Products Committee (BPC) of the European Chemicals Agency (ECHA), in order to boost replicability. The project will make a special contribution to the EU Biocides Product Regulation, especially regarding the precautionary principle (taking into account metabolites of ecotoxicological concern); the promotion of low-risk substances (depending on their ecotoxicological profile); and the reduction of animal testing (in common with Directive 2010/63/EU, which revised Directive 86/609/EEC on the protection of animals used for scientific purposes). Expected results: A platform based on an open-access modular decision-support tool to assess the ecotoxicological properties of biocides and their potential metabolites; Identification of 10% of the most critical biocides based on PBT (persistent, bioaccumulative and toxic) and CMR (toxic for reproduction) criteria; Identification of molecular features responsible for the ecotoxicity of the most critical biocides; Identification of safe moieties (a part or functional group of a molecule), which can be used within the planning of safer biocides; Raised awareness about the use of biocidal products, targeting consumers for the responsible and sustainable use of biocides; and A structured compendium of free webinars and workshops to support the training of end-users and stakeholders for the sustainable use of biocides.