Apparatus and method for high spatial resolution imaging of a sample's structure, including a diffraction-limited resolution volume with a plurality of dye molecules which can be switched between different states and have a distribution density which is greater than the inverse of the diffraction-limited resolution volume, where at least one state is fluorescing, the fluorescence being collected by an objective lens and imaged on a spatially resolving detector by an optical system. At least one light source provided for emitting a switching radiation and for emitting an excitation radiation. At least one of the light sources is arranged to radiate through the sample, and a switching and/or fluorescence excitation of the dye molecules is carried out. The switching is a photoactivation or a photodeactivation of the dye molecules. A focusing arrangement is provided for switching and/or for excitation to generate a line-like illumination region extending in a direction of illumination.