Physical laws set natural limits on how much nature wishes to reveal itself. This is appropriate for light where diffraction of light sets a limit on the resolvable distance. Another limitation is associated with the size of the smallest functional unit of matter (i.e., single molecule) that sets the length scale for investigation. Such limitations on both the probe (here, light) and the matter hold scientific progress, and our understanding of natural processes at the fundamental scale (single molecule). This is true for the field of optical microscopy and related sciences, where the diffraction limit on the resolution has ceased its progress for centuries. The years 1990s and 2000s saw the emergence of three distinct classes of microscopy techniques (STED, SMLM, and SIM) that surpassed the classical limit. Among them, single molecule localization microscopy techniques (fPALM, PALM, and STORM) could reveal the true nature of single molecules. These techniques, for the first time, revealed the nanoscopic world of life processes, including how proteins are manufactured, their interactions, biological functions, and so on. These and many other fundamental processes occurring at single molecule length scales have profound implications in wide research disciplines ranging from natural to engineering sciences.