The Water Framework Directive 2000/60/EC sets out a European strategy preventing water pollution. The directive requires periodical preparation of a list of substances, so called priority substances, which present a significant risk to, or via, aquatic environment. JRC staff, together with the UK Environment Agency, evaluated over two thousand substances put forward by Member States, stakeholders and NGOs, out of which 78 substances were classified as of "high concern" based on their hazard and plausible exposure scenarios. The novelty of the approach used is based on the use of computational methods, which allows for the analysis of a considerable amount of chemicals in a fast way and considers also chemicals that are not routinely monitored in the environment. The development, assessment and application of computational (in silico) methods to support the implementation of EU chemicals policy (including the safety assessment of industrial chemicals, chemicals in consumer products, pesticides and biocides) is one of the activities of the JRC-IHCP (Institute for Health and Consumer Protection). These methods, sometimes referred to as “non-testing methods”, can be used to reduce our reliance on experimental testing, and in particular animal testing.