Abstract: In an optical telecommunications system using wave division multiplexing (WDM) a wavelength add-drop multiplexer (WADM) has an array of adjacently connected filters, each for selecting light of a respective wavelength being transmitted in a multiplexed data stream along an optical fibre, each of the array of filters being adjacently connected to a portion of the array which is a nominally transparent filter and having means for moving the array so that movement between filters in use occurs via the nominally transparent filter portion of the array. Further, in a method of operating a WADM the transition between two filters of an array of filters each for selecting light of a respective wavelength being transmitted in a multiplexed data stream along an optical fibre occurs by way of a nominally transparent filter portion of the array.