Hell, S. W., Dyba, M. and Jakobs, S.
Concepts for nanoscale resolution in fluorescence microscopy. Current Opinion in Neurobiology 14: 599-609 (2004).

Spatio-temporal visualization of cellular structures by fluorescence microscopy has become indispensable in biology. However, the resolution of conventional fluorescence microscopy is limited by diffraction to about 180 nm in the focal plane and to about 500 nm along the optic axis. Recently, concepts have emerged that overcome the diffraction resolution barrier fundamentally. Formed on the basis of reversible saturable optical transitions, these concepts might eventually allow us to investigate hitherto inaccessible details within live cells.