Abstract: An electromagnetic filter assembly includes complimentary first and second blocks (1) and (2) defining two conjoined sections (10), (11) of an electromagnetic waveguide duct. A shim-like electromagnetic filter element (3) is held between the blocks and contains a series of holes (14) defining bridges (15) which traverse the waveguide duct to form poles of an electromagnetic filter. The filter element includes integral tuning projections (20) which extend into waveguide duct between the bridges. By adjusting the position of the filter element (3) between the blocks (1), (2) it is possible to vary the extent to which the tuning projections (20) project into the waveguide duct. The arrangement thus facilitates the repeated, continuous adjustment and variation of the electrical characteristics of devices such is filters, diplexers and multiplexers.