Incineration of waste is increasing in the EU. However, in the incineration process, both fly and bottom ash materials are generated as waste that requires further action. A common goal throughout Europe is to find ways to utilize ash materials in an environmentally and economically efficient manner in accordance with the current legislation. This legislation is the Waste Framework Directive (WFD) which lists essential properties (H-criteria) to classify waste, as hazardous or not. Of these criteria, ecotoxicity (H-14) should be classified based on the wastes’ inherent hazardous properties. The WFD further states that this classification should be based on the Community legislation on chemicals (the CLP Regulation). Today, there are no harmonized quantitative criteria for the H-14 classification in the WFD, but there is a proposal from the EU on a computing model that summarizes all the measured elements classified as ecotoxic in the solid material. However, there may be a poor relationship between the theoretical ecotoxicity, based on analysed individual elements, and their actual contribution to the measured total toxicity. Therefore, to reduce the risk of incorrectly assessing the hazard potential, the overall aim of this doctoral Thesis was to develop a scientifically well-founded basis for the choice of leaching methodology and ecotoxicity testing for the H-14 classification of ash materials in Europe. In Paper I, different ash materials were classified, two leaching methods were compared and the sensitivity as well as the usefulness of a selected number of aquatic ecotoxicity tests were evaluated. Paper III and IV studied different leaching conditions, relevant for both hazard classification and risk evaluation of ash. Moreover, all four papers investigated potentially causative ecotoxic elements in the ash leachates. The results from this Thesis show that elements not classified as ecotoxic in the chemical legislation have a significant influence on the overall toxicity of the complex ash materials and will be considered if using the approach with ecotoxicity tests on ash leachates, but not if using the computing model. In addition, the approach of comparing chemically analysed elements in the solid ash with literature toxicity data for the same elements systematically over-estimates the hazard potential. This emphasizes the importance of using leaching tests in combination with ecotoxicity tests for the ecotoxicity classification of ash materials, at least if the aim is to fully understand the inherent hazard potential of the ash. To conclude, the recommendation for H-14 classification of ash is that leachates should be prepared using the leaching test and conditions evaluated in Paper III and that the generated leachates should be tested in a battery of test organisms representing a wide range of biological variation and different routes of exposure. This classification proposal has support in the CLP Regulation and contributes to harmonizing the waste and chemical legislation.