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In this paper physical concepts that radically break the diffraction barrier in focusing fluorescence microscopy has been discussed. They share a common strategy: exploiting selected molecular transitions of the fluorescent marker to neutralize the limiting role of diffraction. The first viable concept of this kind was Stimulated Emission Depletion (STED) microscopy. In its simplest variant, STED microscopy uses a focused beam for fluorescence excitation, along with a red-shifted doughnut-shaped beam for subsequent quenching of fluorescent molecules by stimulated emission.