A combustor for a gas turbine engine comprising: a combustion chamber; at least one fuel spray nozzle operable to deliver a fuel-air mixture into the combustion chamber, wherein during operation of the gas turbine engine the fuel-air mixture is combusted in the combustion chamber, thereby producing a combustion flame; a microwave generator coupled to a waveguide arranged to guide microwaves from the microwave generator into the combustion chamber such that the microwaves are incident on at least a portion of the combustion flame; and a detector operable to detect at least a portion of the microwaves reflected by the combustion flame and/or atomised fuel droplets.