A method of tuning a microwave or RF circuit is given which comprises the steps of taking a microwave or RF circuit (20) located in a casing (57), said casing comprising a housing portion (32) and a window portion (50), said window portion (50) being substantially conducting at microwave/RF frequencies, and comprising at least one area that is substantially transparent at optical frequencies and, directing a laser beam (66) onto said microwave or RF circuit through said window portion (50) so as to alter the material properties of selected areas of said microwave or RF circuit (20). This permits microwave and RF circuits, including microwave filters, to be tuned without the need for tuning screws.