Fly ashes are one of the waste products from incineration of coal, urban or industrialresidues and biomass to mention a few. Fly ash is the general term used for incinerationby products, namely particulates and flue gas additives captured from the flue gas stream.They are considered hazardous due to their content on contaminants, especially heavymetals. Fly ashes constitute an environmental problem both due to their hazardousnessand instability, needing previous stabilization before disposal in landfill.Besides the hazardousness, fly ashes also possess features that could be furthervalorised in uses as different as soil amendment, ceramics or concrete. However, due tocontaminant content of fly ash, the reuse of such a waste should be always in accordancewith the precautionary principle and the uncertainty of the contaminants behaviourthroughout time.The present study is dedicated to the characterization of different fly ashes, itshandling and the minimization of heavy metals leachability to the surroundingenvironment. Fly ashes from straw (ST) combustion and from co-combustion of woodwith fuel oil and natural gas (CW) were analyzed.The theoretical approaches for fly ash reuse vary on a specific point: (i) remediation,where the contaminants are extracted from the solid porous fly ash; and (ii)stabilization/solidification (S/S), where contaminants are embedded in a solid matrix.In order to understand the origin of fly ashes and its formation, a review on theincineration process and its efficiency is given. According to literature, fly ashes aredependent of incineration parameters such as temperature, and, most importantly, thefuel/waste. Furthermore, fly ash is defined, the electrodialytic process for its remediationis described and a possible reuse is assessed, always considering the precautionaryprinciple.A series of 4 electrodialytic experiments, with 3 replications each, are described: twowith ST ashes and two with CW ashes. A statistical study was carried out to determinewhich variables ("Ash type", "Duration", "Final pH", "Acidification" and "Dissolution")were the most significant for EDR efficiency. Cd, Cr, Cu, Ca and Zn migration velocityand acceleration were characterized using a biregressional design.