Throughout the 20th century it was widely accepted that, at the end of the day, a light microscope relying on conventional lenses (far-field optics) cannot discern details that are finer than about half the wavelength of light (>200 nm). However, in the 1990s, it was discovered  that overcoming the diffraction barrier is realistic and that fluorescent samples can be resolved virtually down to molecular dimensions. Here I discuss the simple yet powerful principles that allow neutralizing the resolution-limiting role of diffraction and have led to the emergence of far-field optical ‘nanoscopy’ as a field[2,3]. In a nutshell, features residing closer than the diffraction barrier are prepared in different molecular (quantum) states so that they are distinguishable for a brief detection period. As a result, the resolution-limiting role of diffraction is overcome, and the interior of transparent samples such as living cells and tissues can now be imaged with nanoscale resolution using focused light in 3D.
Finally, I will show that the in-depth description of these basic principles spawns new nanoscopy concepts such as MINFLUX  which has obtained the ultimate optical (super)resolution: the size of a molecule .
 Hell, S.W., Wichmann, J. Breaking the diffraction resolution limit by stimulated emission: stimulated-emission-depletion fluorescence microscopy. Opt. Lett. 19, 780-782 (1994).
 Hell, S.W. Far-Field Optical Nanoscopy. Science 316, 1153-1158 (2007).
 Hell, S.W. Microscopy and its focal switch. Nat Methods 6, 24-32 (2009).
 Balzarotti, F., Eilers, Y., Gwosch, K. C., Gynnå, A. H., Westphal, V., Stefani, F. D., Elf, J., Hell, S.W. Science 355, 606-612 (2017).