The invention relates to a method for controlling a temperature distribution in a heat exchanger, in which an actual temperature distribution in the heat exchanger is measured by means of at least one optical waveguide arranged in the heat exchanger, in particular in the form of a glass fiber, light being launched into the optical waveguide and light that is scattered in the optical waveguide being evaluated for determining the actual temperature distribution, and at least one flow of a fluid medium that is carried in the heat exchanger being controlled in such a way that the actual temperature distribution is made to approximate a pre-defined target temperature distribution. The invention also relates to a device for carrying out a method for controlling a temperature distribution in a heat exchanger.