Bottom ash is the residual solid waste collected in waste-to-energy plants, where it accounts for about 25% wt of incinerated municipal solid waste. The chemical composition and physical properties of bottom ash make it worth of recycling metals and inert material for construction purposes. In 2014, 88 M tonnes of mixed waste were treated in EU waste-to-energy plants, generating 18 Mt of bottom ash, made of 80–85% mineral fraction, 10–12% ferrous and non-ferrous metals and a residual amount of unburnt materials. State-of-the-art technologies in countries with a long incineration history are able to separate the different components (metals are recycled in smelters while the mineral fraction is used for construction purposes), but the optimisation of the recovery of metals (particularly non-ferrous ones) and of the mineral fraction needs further efforts.The H2020 project “BASH-TREAT - Optimization of bottom ash treatment for an improved recovery of valuable fractions”, won the call ERA-MIN2 “Research and Innovation Programme on Raw Materials to foster Circular Economy” (May 2018-May 2021), is coordinated by Technical University of Hamburg and involves DIATI Politecnico di Torino as partner.