Abstract: A system for controlling gain of an optical amplifier that is disposed in a WDM optical communication network for amplifying an input WDM signal to produce a corresponding amplified output WDM signal is operative for measuring average optical powers of the input WDM signal over first and second wavebands, each covering a wavelength range that includes at least one wavelength channel of the WDM network and is operative for measuring the average optical powers of the amplified output WDM signal over the first and second wavebands. The system further determines gain values over the first and second wavebands from the average input and output powers and adjusts a variable of the optical amplifier to ensure that the difference between the gain values is maintained to within a predetermined value thereby ensuring a substantially flat gain slope profile.