Abstract: This invention generally relates to a method of controlling emission wavelength of a light emitting device, and a system for wavelength control of a light emitting device, for example for wavelength stabilisation of a laser diode. One method of controlling emission wavelength of a light emitting device comprises: determining an indication of series resistance of the device, the series resistance comprising ohmic resistance of the device; providing a current through the device to maintain light emission of the device; measuring a forward voltage of the device during said light emission, the forward voltage being across an impedance comprising impedance of an active region of the device and the series resistance; determining an indicator of active region voltage on the basis of the measured forward voltage and the determined indicator of series resistance; and controlling a temperature of the device on the basis of the determined indicator of active region voltage.