Subject to the diffraction limit, the resolution of conventional optical microscopy is constrained to about 200 and 500 nm in the lateral and axial planes, respectively. The advantage of optical microscopy in the life sciences over electronic microscopy, especially fluorescence microscopy, drives scientists to develop novel "hacks" to reach nanoscale resolutions by optical means. In this review, three aspects of the techniques are discussed: (1) lateral super-resolution; (2) axial super-resolution; (3) super-resolution in three dimensions. The principles of how the methods achieve the cross-barrier resolution are discussed, and recent advances in current techniques are described. With these methods, the use of fluorescence microscopy is growing quickly toward a new era: fluorescence nanoscopy that will reveal 2 orders of magnitude more information on cellular structure and dynamics.
