A reagent dosing system for dosing a reagent into the exhaust gas stream of an internal combustion engine includes a reagent tank for storing a supply of reagent; an injector module including an atomising dispenser and a positive-displacement metering pump which draws reagent from the reagent tank and delivers it to the dispenser; a supply line coupling the reagent tank to the injector module; a dosing control unit operable to control the injector module to inject reagent into the exhaust gas stream; and an additional priming pump arranged, in use, to urge reagent along the supply line toward the injector module under selected conditions.