A variable gain optical amplifier comprises an EDFA for amplifying optical signals at different wavelengths and a pump driver 14 for optically pumping the EDFA to provide optical gain. An input detector 2 is provided for monitoring the power P.sub.in of input signals to the EDFA, and an output detector 3 is provided for monitoring the power P.sub.out of output signals from the EDFA. A gain control arrangement is provided for supplying a drive signal to the pump driver 14 to control the optical gain including a feed forward arrangement 20, 21, 22, 23 for supplying a feed forward signal dependent on the monitored input power P.sub.in, and a feed back arrangement 5, 6, 7, 8, 9, 30 for supplying a feed back signal dependent on the monitored output power P.sub.out. In order to ensure rapid gain control the feed back arrangement comprises an adaptive proportional-integral (PI or PID) controller 30 for controlling the optical gain at a required gain set point in accordance with proportional and integral control coefficients K.sub.p and K.sub.i corresponding to a required gain profile, at least one of which is dynamically variable in dependence on the monitored output power P.sub.out, the output signal from the controller 30 and the feed forward signal being added in an adder 31 to produce the drive signal for the pump driver 14.