A MEMS device comprises first and second opposing electrode arrangements (22,28), wherein the second electrode arrangement (28) is electrically movable to vary the electrode spacing between facing sides of the first and second electrode arrangements. At least one of the facing sides has a non-flat surface with at least one peak and at least one trough. The height of the peak and depth of the trough is between 0.01t and 0.1t where t is the thickness of the movable electrode.