A method of treating sewage sludge comprises a pathogen reduction stage in which raw sludge is incubated for a predetermined incubation period at a temperature in the mesophilic temperature range. The sludge is incubated in a plurality of sequential vessels (1a,1b,1c) comprising an upstream vessel (1a) which receives the raw sludge to be incubated and a downstream vessel (1c) which discharges the incubated sludge. The sludge is fed from the upstream vessel (1a) to the downstream vessel (1c), via any intermediate vessels (1b), and subsequently discharged after spending a predetermined residence time in each vessel, the total residence time being said predetermined incubation period.