Ground state depletion microscopy followed by individual molecule return (GSDIM) is a super-resolution technique based on single molecule localization (Localization Microscopy). To localize single molecules and create a high resolution image the ensemble of overlapping fluorophores (in a diffraction-limited setup) has to be broken up. Individual fluorophores must be temporally "separated" to allow high precision detection of single molecules.
This can be achieved by using high power lasers to transfer fluorophores into long-lived "off states" – a non-fluorescent molecule state. Single fluorophores return stochastically from the off state and emit bursts of photons, which are recorded. The position of the fluorophore is determined using a software algorithm. Based on this list of coordinates a super-resolution image is reconstructed.
This tutorial will explain the molecular basics of GSDIM (the even shorter abbreviation GSD is nowadays in use for this technology, too).