||Optical microscopy has been a favorite tool of biologists for unlocking life’s mysteries since the 20th century. Using visible light to investigate fluorescently labeled biological samples takes advantage of a noninvasive probe character and a selective and specific marking of investigated species. Adding high magnification objectives allows one to observe small objects; adding high sensitive detectors allow investigation of single-molecule concentrations. In this talk, we shall discuss the path optical microscopy has taken in the last decade. We shall see examples of how fluorescence on-off switching allows localizing and analyzing labeled protein molecules at the nanoscale. Meaning that the Abbe’ diffraction barrier, constraining the optical resolution of standard microscopy to about half the wavelength, has been possible to circumvent. Especially, application of the emerging super-resolution STED fluorescence microscopy technique will be highlighted.