An igniter assembly for a combustion system of a gas turbine engine is provided. The igniter assembly includes an igniter plug having a tip which, in use, is located at an aperture in a wall of a flame-tube of the combustion system such that the tip can provide an electrical discharge to ignite an air-fuel mixture within the flame-tube. The igniter assembly further includes a substantially annular igniter boss which locates on the wall of the flame-tube at the aperture to encircle the tip. The igniter boss has a slot formed therein at one side of the tip. An entrance of the slot receives air from outside the flame-tube and an exit of the slot delivers the air as a sheet-shaped flow over the tip. The slot is tapered such that the thickness of the slot narrows from its entrance to its exit.