The present invention provided a method of manufacturing a duct for a gas turbine engine or the like from sheet metal material; the method comprising the steps of: fabricating a duct from sheet material (10,12,14) including rolling or folding metal sheet along ruled lines to form a ruled surface geometry approximately corresponding to at least part of a desired irregular final duct geometry; welding corresponding rolled or folded parts of the duct skin together and positioning the duct on a heat treatment fixture (30) having a shape corresponding to the said desired irregular duct geometry; the material of the heat treatment fixture having a greater thermal expansion coefficient than the duct material; heating the duct and fixture substantially to a stress relieving temperature of the duct material such that differential thermal expansion of the duct and fixture at the said stress relieving temperature causes the duct to distort and adopt the shape of the exterior surface of the fixture on which the duct is located.