Stimulated emission depletion (STED) is a powerful super-resolution technique that allows visualization of sub-diffraction features and biological structures well beyond the diffraction limit ( \({\sim } \lambda /2\) ) set by classical light. Traditional light microscopes are limited by the diffraction of light, which prevents them from resolving features smaller than this limit. STED microscopy overcomes this by using a dual-beam illumination, with a regular Gaussian beam to excite (“switch-on”) molecules emitting fluorescence in a diffraction-limited Gaussian spot and a doughnut beam to “switch-off” or quench molecules at the periphery of the spot. This allows controlled switching OFF of molecules thereby shrinking the system PSF to as small as possible (theoretically), thereby improving the resolution by many-fold. Over the years, the technique has advanced beyond traditional STED, and many variants (two-photon STED, time-gated STED, MINSTED, MINFLUX) were invented that improved resolution and provided additional features (3D imaging, depth imaging, and multi-organelle imaging). This has pushed PArticle Resolution shift (PAR-shift) towards the actual size of a single molecule. PAR-shift achieved by STED-based microscopy techniques has enabled the study of protein-protein interaction, visualization of biological structures, and nanoscale processes in living cells in real-time.