Since the 19th century it has been widely accepted that a light microscope cannot see details that are finer than half the wavelength of light (>200 nm). However, in the 1990s it was discovered that this diffraction resolution barrier can be effectively overcome, such that fluorescent features can be resolved virtually down to molecular dimensions, using regular lenses. Here we discuss the simple yet powerful physical principles that allowed us to break the diffraction resolution limit, with special emphasis on STED microscopy. Besides, we exemplify the relevance of this rapidly emerging ‘optical nanoscopy’ to various fields, in particular the material and the life sciences, where the interior of cells and tissues can now be investigated non-invasively at the nanoscale [1-3].

1. Hell, S. W. Far-field optical nanoscopy, in Single Molecule Spectroscopy in Chemistry (ed A. Gräslund, Rigler, R., Widengren, J.) 365 – 398 (Springer, 2009).
2. Grotjohann, T. et al. Diffraction-unlimited all-optical imaging and writing with a photochromic GFP. Nature 478, 204-208 (2011).
3. Wildanger, D., Maze, J. R. & Hell, S. W. Diffraction unlimited all-optical recording of electron spin resonances. Phys Rev Lett 107, 017601 (2011).