A method for the treatment of fly ash obtained from an incineration process of a waste incineration plant, in particular for municipal solid waste, whereby fly ash is separated from the incineration process. Furthermore, a method for the operation of a waste incineration plant, in particular for municipal solid waste or the like. The process for the treatment of fly ash is further characterized in that metals and/or metal containing compounds, in particular heavy metals and/or heavy metal containing compounds, are separated from the fly ash, which is separated from the combustion process and preferably non-fractioned, in a separation step and subsequently the fly ash reduced by the metals and/or metal containing compounds is, preferably dosed, mixed with or added to the waste to be incinerated so that the mineral parts of the fly ash, reduced by its metals and/or metal containing compounds, are returned to the combustion process.