Far-field optical fluorescence microcopy has become an essential tool in life science for a long time largely owing to its unique capability to provide noninvasive, three-dimensional (3D) imaging inside cells. However, resolution of a traditional wide-field optical microscopy is limited to about 230 nm laterally and 1000 nm axially, due to the diffraction-limit of light. Resolution improvement is urgently demanded because molecule-scale dynamics and structures are to be revealed inside living cells in today’s life science. So far, many scientists have proposed a significant amount of novel methods in order to enhance resolution of far-field optical imaging. For example, lateral resolution of approximately 100 nm has been achieved by use of structured illumination, whereas the axial resolution has been enhanced 5～10-fold using a standing wave produced by two beams propagating in opposite directions. Nevertheless, diffraction barrier was not broken in these cases until nonlinear optical effects were introduced into optical fluorescence microscopy. As an example, the use of a nonlinear optical effect, namely, simulated emission depletion microscopy has resulted in a 3D resolution of 30～50 nm. Furthermore, the barrier of diffraction-limit can also be broken by novel technologies based on fluorescence resonance energy transfer and high-accuracy localization of fluorophores, by which molecules can be positioned with a resolution of several nanometers.