A method (10, 30, 90, 110, 120) of configuring an optical network terminal of a wavelength division multiplexed passive optical network, the method comprising: a. sequentially setting a transmission wavelength of a tunable optical filter at the optical network terminal to one or more wavelengths of a pre-selected plurality of transmission wavelengths until a said wavelength is identified for which no downstream optical signal from an optical line terminal of the wavelength division multiplexed passive optical network is detected; characterized in that the method further comprises: b. determining whether a transmitter operable at said identified transmission wavelength is present at the optical line terminal and is available to be assigned to communicate with the optical network terminal by generating an upstream optical signal and transmitting the upstream optical signal to the optical line terminal, waiting for a time at least as long as a round trip time from the optical network terminal to the optical line terminal and back, and determining whether a further downstream optical signal generated and transmitted by the optical line terminal has been received at the optical network terminal, wherein a said further downstream optical signal is determined as having been received if a loss of signal alarm having a low alarm condition is received at the optical network terminal; c. if a said transmitter is determined to be present and available, maintaining the transmission wavelength of the tunable optical filter at said identified transmission wavelength and assigning the optical network terminal to the optical line terminal as a subscriber at said identified transmission wavelength; and d. if a said transmitter is determined to be one of not present and not available, recommencing the method at step a. for one or more further wavelengths of said pre-selected plurality of wavelengths. a. sequentially setting a transmission wavelength of a tunable optical filter at the optical network terminal to one or more wavelengths of a pre-selected plurality of transmission wavelengths until a said wavelength is identified for which no downstream optical signal from an optical line terminal of the wavelength division multiplexed passive optical network is detected; characterized in that the method further comprises: b. determining whether a transmitter operable at said identified transmission wavelength is present at the optical line terminal and is available to be assigned to communicate with the optical network terminal by generating an upstream optical signal and transmitting the upstream optical signal to the optical line terminal, waiting for a time at least as long as a round trip time from the optical network terminal to the optical line terminal and back, and determining whether a further downstream optical signal generated and transmitted by the optical line terminal has been received at the optical network terminal, wherein a said further downstream optical signal is determined as having been received if a loss of signal alarm having a low alarm condition is received at the optical network terminal; c. if a said transmitter is determined to be present and available, maintaining the transmission wavelength of the tunable optical filter at said identified transmission wavelength and assigning the optical network terminal to the optical line terminal as a subscriber at said identified transmission wavelength; and d. if a said transmitter is determined to be one of not present and not available, recommencing the method at step a. for one or more further wavelengths of said pre-selected plurality of wavelengths.