In an apparatus and a method for generating monochromatic X-ray radiation, an X-ray source, a monochromator and a slit collimator are arranged relative to one another such that X-rays of a specific energy among the X-rays emanating from the X-ray source are reflected at the monochromator and emerge through the slit of the slit collimator as a fan-shaped beam of monochromatic X-radiation. For scanning an examination subject with the X-ray beam, the monochromator is adjustable relative to the X-ray source and the slit collimator such that the condition for the reflection angle required for the reflection of X-rays of the specific energy at the monochromator remains substantially satisfied during the adjustment, and essentially only X-rays of the specific energy pass through the slit of the slit collimator.